Vanish let the character hide and can't trace by non-magical means.

Does Vanish auto success stealth check if there isn't a magical detection?

What is "Vanish" in reference to?

Feat? Magic Item? Subclass Feature?

A little context will help.

 Vanish says "you can use the Hide action as a bonus action on your turn."

The Hide Actions says "You roll a stealth check"

So yes you still need to roll.  Not being able to be tracked doesn't mean that you can't still be perceived.  It means when you are traveling you leave no tracks that someone could make a Survival Check to search for.

Example: You try to hide behind a bunch of boxes in the middle of a fight.  That stealth check would be for enemies to notice your head poking out from over them, or your right foot that you didn't quite get behind the box.

it's a feature for the ranger class

Vanish is a level 14 Ranger feature:

Vanish

Starting at 14th level, you can use the Hide action as a bonus action on your turn. Also, you can’t be tracked by nonmagical means, unless you choose to leave a trail.

I read "tracked" in this feature to refer to Tracking in the DMG.: the art of following the tracks of creatures to find them using Survival. 

Tracking

Adventurers sometimes choose their path by following the tracks of other creatures — or other creatures might track the adventurers! To track, one or more creatures must succeed on a Wisdom (Survival) check. You might require trackers to make a new check in any of the following circumstances:

• They stop tracking and resume after finishing a short or long rest.
• The trail crosses an obstacle, such as a river, that shows no tracks.
• The weather conditions or terrain changes in a way that makes tracking harder.

The DC for the check depends on how well the ground shows signs of a creature’s passage. No roll is necessary in situations where the tracks are obvious. For example, no check is needed to track an army advancing along a muddy road. Spotting tracks on a bare stone floor is more challenging, unless the creature being tracked leaves a distinct trail. Additionally, the passage of time often makes tracks harder to follow. In a situation where there is no trail to follow, you can rule that tracking is impossible.

The Tracking DCs table offers guidelines for setting the DC or, if you prefer, you can choose a DC based on your assessment of the difficulty. You can also grant advantage on the check if there’s more than one set of tracks to follow, or disadvantage if the trail being followed passes through a well-trafficked area.

On a failed check, the character loses the trail but can attempt to find it again by making a careful search of the area. It takes 10 minutes to find a trail in a confined area such as a dungeon, or 1 hour outdoors.

Tracking DCs

Ground Surface	DC
Soft surface such as snow	10
Dirt or grass	15
Bare stone	20
Each day since the creature passed	+5
Creature left a trail such as blood	−5

That is not the same thing as searching or "noticing" to find a hidden creature. That is never called "tracking", and what we know about that comes from the PHB:

Hide

When you take the Hide action, you make a Dexterity (Stealth) check in an attempt to hide, following the rules for hiding. If you succeed, you gain certain benefits, as described in the "Unseen Attackers and Targets" section later in this chapter.

Stealth

Make a Dexterity (Stealth) check when you attempt to conceal yourself from enemies, slink past guards, slip away without being noticed, or sneak up on someone without being seen or heard.

HIDING

The DM decides when circumstances are appropriate for hiding. When you try to hide, make a Dexterity (Stealth) check. Until you are discovered or you stop hiding, that check's total is contested by the Wisdom (Perception) check of any creature that actively searches for signs of your presence.

You can't hide from a creature that can see you clearly, and you give away your position if you make noise, such as shouting a warning or knocking over a vase. An invisible creature can always try to hide. Signs of its passage might still be noticed, and it does have to stay quiet.

In combat, most creatures stay alert for signs of danger all around, so if you come out of hiding and approach a creature, it usually sees you. However, under certain circumstances, the DM might allow you to stay hidden as you approach a creature that is distracted, allowing you to gain advantage on an attack roll before you are seen.

Passive Perception. When you hide, there's a chance someone will notice you even if they aren't searching. To determine whether such a creature notices you, the DM compares your Dexterity (Stealth) check with that creature's passive Wisdom (Perception) score, which equals 10 + the creature's Wisdom modifier, as well as any other bonuses or penalties. If the creature has advantage, add 5. For disadvantage, subtract 5.

For example, if a 1st-level character (with a proficiency bonus of +2) has a Wisdom of 15 (a +2 modifier) and proficiency in Perception, he or she has a passive Wisdom (Perception) of 14.

What Can You See? One of the main factors in determining whether you can find a hidden creature or object is how well you can see in an area, which might be lightly or heavily obscured as explained in chapter 8, “Adventuring.”

Perception

Your Wisdom (Perception) check lets you spot, hear, or otherwise detect the presence of something. It measures your general awareness of your surroundings and the keenness of your senses. For example, you might try to hear a conversation through a closed door, eavesdrop under an open window, or hear monsters moving stealthily in the forest. Or you might try to spot things that are obscured or easy to miss, whether they are orcs lying in ambush on a road, thugs hiding in the shadows of an alley, or candlelight under a closed secret door.

 Thanks for the answers so far.

A follow-up question: Can I use Vanish under any circumstances i.e. in an open area during a sunny day?

Can you Hide in an open area on a sunny day?

To hide requires some ability to obscure yourself.  Vanish is not a magical thing.  It just makes you really good at preventing people from picking up on your trail.  So if you were to walk out of that open area into a forest, no one knows which way you went.  But you can't just hide if there is literally nothing to hide behind or in.

Quote from Frozen_Fruit >>

 Thanks for the answers so far.

A follow-up question: Can I use Vanish under any circumstances i.e. in an open area during a sunny day?

 Chicken_Champ answered your question up above.

Hmm, I think of the vanish feature more like the green hag's invisible passage feature, which uses similar wording with regard to tracking. "While invisible, she leaves no physical evidence of her passage, so she can be tracked only by magic." In that sense, if you pass your stealth check and become hidden, it would allow you to avoid being noticed via perception checks.

At least I feel like that is the intention behind it.

Thx for the info. It also helps to answer the 2nd question. There are no "invisible" in Vanish, so I guess I can't hide with Vanish without something to cover.

Quote from TexasDevin >>

Hmm, I think of the vanish feature more like the green hag's invisible passage feature, which uses similar wording with regard to tracking. "While invisible, she leaves no physical evidence of her passage, so she can be tracked only by magic." In that sense, if you pass your stealth check and become hidden, it would allow you to avoid being noticed via perception checks.

At least I feel like that is the intention behind it.

 If there wasn't cleared defined rules for tracking someone maybe.  However the feature states clearly, cannot be tracked by non magical means.  Tracking something is a Survival check, not perception.  If you hide behind something, someone can still notice you (Perception check) but if they don't see you and you move somewhere else, they will not be able to roll a Survival check to follow your tracks.

Quote from Frozen_Fruit >>

Thx for the info. It also helps to answer the 2nd question. There are no "invisible" in Vanish, so I guess I can't hide with Vanish without something to cover.

  You want to look at the Feature "Hide in Plain sight" if you want to be able to hide anywhere:

Hide in Plain Sight

Starting at 10th level, you can spend 1 minute creating camouflage for yourself. You must have access to fresh mud, dirt, plants, soot, and other naturally occurring materials with which to create your camouflage.

Once you are camouflaged in this way, you can try to hide by pressing yourself up against a solid surface, such as a tree or wall, that is at least as tall and wide as you are. You gain a +10 bonus to Dexterity (Stealth) checks as long as you remain there without moving or taking actions. Once you move or take an action or a reaction, you must camouflage yourself again to gain this benefit.

So if we are drawing a distinction between the ability to be detected and the ability to be tracked, does that mean that pass without trace also makes the distinction? And if so, would it make someone unable to be tracked except by magical means AND ALSO separately masking you and your companions from detection... by any means?

There already seems to be a distinction in the spell description. The bonus is +10 to a Stealth check that is countered the same way as always, with a Passive Perception or an active Search action if it is justified. You can't be tracked by mundane methods. Two different thoughts, one sentence.

Well then maybe it's me who has been playing it wrong this whole time. I am curious if other people have been treating "cannot be tracked by nonmagical means" as "cannot be perceived by nonmagical means."

If you couldn't be perceived by non-magical means, you wouldn't need a + 10 Stealth bonus, right?  : P

Quote from TexasDevin >>

Well then maybe it's me who has been playing it wrong this whole time. I am curious if other people have been treating "cannot be tracked by nonmagical means" as "cannot be perceived by nonmagical means."

 No.

The Mind Spike spell would work to track you, and would be an example of magical “tracking” just as a comparison to the “non-magical” kind