The party is on some sort of extended vacation, staying in an inn/bar. A frequent visitor is a tall, dark, suave, charming man dressed in formal evening wear, accompanied by a different woman every time. He comes in every 2nd or 3d night. He always orders bloody marys and doesn't drink them. He is quite wealthy and very pleasant. There is something almost magnetic about him. He has fascinating eyes. (DM should do everything he can to make it believable that he could be a vampire, despite the unusual setting (city)).
Either he charms (charm gaze) a female party member and takes her away, or a beautiful dancer comes in looking for her missing sister, who was last seen coming to this bar with the tall,dark gentleman. She tries to convince a party member to help her look for her sister being seductive about it. Both are eventually charmed by the Gentleman. In any case, make a party member disappear into this Gentleman's lair.
He has a gothic style house in a nice part of town. There is nothing obviously amiss here. If the party asks around, this guy is a pillar of society, a kind, philanthropic fellow, well respected by his peers. He runs a magic shoppe. He is a mid-level wizard with a head for business, who gave up adventuring to start a business.
His house looks just like a vampires house might look (black velvet curtains, etc). He has a private sanctuary inn his basement, the only entrance to which is a rune-encrusted door (trapped or enchanted in any way appropriate to the party). He supposedly has a chapel down there, but really has a large complex, where various vampiric rituals, and all- night parties take place. All of the missing people have been charmed into believing that they have been turned into slave vampires. They will aid their master if at all possible.
The party must break in and forcibly take their companion away from this place. Again, make the evidence somewhat contradictory whether the Gentleman is a vampire or not. Most evidence should say yes, but make some things contradict this.
The gentleman has a cursed ring of the vampire, a powerful evil artifact which makes him believe he is a vampire and gives him many of the powers of a vampire, as well as some of the drawbacks. Make him dislike things that cause a vampire harm, but don't make it obvious whether is works. Make him have a reflection, but have a dead vampire victim show up. Etc. At the end, have the party realize that he is not a vampire at all but rather is a cursed fellow with an intrinsically good nature.