So, in my homebrew Eberron campaign, the party is consists of the following:

• A Level 4 Firbolg Open Hands Monk
• A Level 4 Mark of Making Human War Wizard
• A Level 5 Warforged Battle Smith Artificer
• Steel Defender, shaped like a snake

They are just going to enter into the Mournland, trying to break into a House Kundarak Bank to retrieve some gold from the abandoned building.

What encounters, or creatures, should they encounter on the way to this location in Northern Cyre, or in the vault?

I think you should have a spinx in that so they can only get to the vault if they get the riddle right.

It's said that natural forces such as decomposition and natural healing just don't happen in the Mournland. Perhaps you could make it so the players do not heal during rests, without magical healing. This is a huge, huge debuff, though, so you should be careful. Instead, you could say that it only effects those who have been in the mournland for a long time. After a while, their wounds no longer heal, and the pain never goes away. They don't need to eat, but they still feel hunger. They can't even die, short of being completely obliterated. If you've read Elantris by Brandon Sanderson before, I imagine something like that. And then there are the monsters. Foul beings warped by magic walk the land, so perhaps you could have something akin to the Shadowfell's sorrowsworn, from MToF. Massive open graves litter the land, full of corpses that refuse to decompose. The perfect place for scavengers, which could have also been warped, becoming horrible monstrosities. I imagine some sort of hulking, ravenlike beast too large to fly, that instead leaps across mounds of bodies, using its stunted wings to glide, and has the ability to mimic any sound, similar to a kenku. And then, of course, there's the undead. Lotsa undead.

As for what they might encounter in the bank and in the vault, you could go two ways with it. Either it could be abandoned, yet still protected by all manners of traps, wards, and constructs, including possibly a mechanical gorgon or two. Or, someone could have claimed the vault and the riches within for themself. Perhaps a maddened survivor of the Mourning, proclaiming himself king and lording over the land, giving orders to his tiny court of survivors, and holding mock trials and executions(everything's mock except the axe). Or maybe it's a greedy beast, like a dragon, a beholder, or something else(somehow weakened, of course, because the party wouldn't be able to take a normal one on). Or maybe someone else has already been there, and taken it elsewhere, yet they've left clues behind them. Or, something else entirely! It's your campaign, after all.

A few bullet points from my notes on a game im running shortly: 

• Living spells are attracted to the party. Seemingly innocuous at first, dancing lights begin to play and float around them, or wild fairie fires, but those in turn attract bigger, badder living spells; fireballs, lightning bolts, etc.
• The players come across a lost human child out in the waters. She is terrified and has no idea how she got there. It turns out there's a pool that reflects an image of Cyre's past, which, for the girl, functioned as a portal through time when she fell in. This may not be immediately apparent, and the party must decide if she is as she seems, or if she is some kind of trap the Mournlands are waiting to spring. 
• Mutated wildlife, you know the drill. 
• A patrol of warforged loyal to the Lord of Blades can be seen and must be either confronted or avoided. 
• A mad mage living in a decrepit tower kidnaps wanderers for experimentation. Players may be kidnapped, or if they avoid capture, may wish to save someone who has been taken.