This is so cool. Can't wait to send some "spies" from Strahd or other vampires to check on my players ;)

Finally, a statblock for my unpaid servant Gizmo.

The immunity to charm makes these servants particularly effective at stymieing the influence of creatures like the fey, aboleths, mind flayers, etc. who control others by charming them. Interesting dynamics could arise (something is happening to the local populace, but Renfield isn't affected).

So excited to run this stat block in the future. 

I really think it is a miss on how these are created.  I know the willing part or charmed is obvious but there has to be some ritual or is the dagger what gives them their abilities?

I mean depending on the lore you go by it's typically the vampire that gives the familiar their abilities, the specifics of how or the ritual is up to the given dm unless the monster manual gives specifics on how they're created as far as the standard settings of dnd are concerned. As for the dagger I'd assume it's either a magic item gifted by the vampire, or a effect applied to all daggers the familiar wields as an effect of the ritual, in order to better acquire people to feed their master when the food is less willing to come along 

So can these guys let vampires in? Like, if a Familiar infiltrates a tavern, then stands ready when their master arrives, can he technically allow a vampire into a residence?

Interesting that they chose the "hostage situation" Vampire Familiar setup, it seems like choosing a more willing familiar would go with the suggestions that they made in the article about cool new monster variants in the MM. Also, I might consider having one of my players be in a situation where becoming a familiar was an option to get out of combat.

 Judging by the picture and by "vampire servants show signs of their vampiric corruption, such as corpse-like complexions, uncanny reflexes, and evidence of their masters' repeated feedings" does this mean they can't turn into a vampire by a regular bite, or that vampires get to choose if their bite inflicts vampirism, or...?

You have to be killed by a vampire's bite attack to become a vampire, I think.