This is what I’m picturing for your costume:
Skinny black jeans
poet shirt black or white
biker boots and black leather jacket
maybe a little guy liner
This part of your OP cracked me up “BTW, this is not a hypothetical question with real vampires; it’s a real question with what I assume will be pretend vampires.” I think your sense of humor will serve you well.
I also often find myself not drinking around people who are drinking. Consider yourself fortunate that there is a theme, because that gives the event some focus besides drinking.
One of the best pieces of advice I’ve heard for situations when you are at a party alone or are shy in groups is to take a proactive role in helping others feel comfortable. Be the one who compliments others on their costume, helps with costumes that need repair or come with awkward parts, offer to take people’s cloaks, etc. Maybe bring a camera if people will want to have pictures taken (I imagine them all dying when the flash goes off…). You can expand this approach into helping run the music, organize the best costume contest, and so on if the host has stuff like that planned.
Dude. You want to be the center of attention? You want to get all the girl’s numbers? You want to have an easy schtick that you can use instead of being nervous?
Dress up as Count Von Count. Going to need a little facepaint, but, hey.
“Von! Von pretty girl!”
Drink Bloody Marys.
Wear a plunging neck line.
Buy a spray can of air freshener. Cover the brand name with a sticker that says “Vampire Spray.” For added effect, spray it on yourself and moan, “It burns! It burns!”
Man the keg.
Great costume idea. Get an old broken keg tap and glue it to your jugular.
You don’t mention when the party is, but if you have time, how about a pair of Lestat or Loius contacts?
Or just look around on there or other sites for a look that suits you. You’ll be the center of attention anywhere you go.
Take a hand mirror and keep looking in it. Pretend you can’t see yourself. Act anguished.
Whatever you do, don’t ask for tea at a vampire party!
Get welder’s goggles and a Victorian laboratory smock. Carry some odd equipment. Claim to be a scientist engaged in a study of vampires.
Wear a strait jacket and go as Renfield. Call everybody else master or mistress.
Serious point. Is this a Vampire Masquerade party, or a Vampire The Masquerade party? There is a… significant difference. One’s a bit of fun, the other is a game people play and can be quite arseholes about.
I can bring you up to speed on most of it, though it’s changed since I was involved.
If this is a Vampire The Masquerade party, I bet a bunch of us can help.
As an Etheric Scientist, I’ve spent some time investigating the so called Cainites.
I know where you can get some candied insects (I can’t guarantee how much life force is still in them).
What’s the difference? 
:dubious:
Vampire-The Masquerade is the name of a role playing game formerly published by White Wolf games. If this is a Vampire The Masquerade event, then these are specific kinds of vampires with a specific world history, specific powers etc.
Learn how to spot porcelain vs plastic fangs. Read up on some designers of porcelain fangs and ask, “Hey are those made by…”
Don’t worry about looking like a poser, goths all think everyone at the party but them and their friends are posers anyway.
This post is obfuscated. move along.
Sparkle, you fool, sparkle!
This is what I was going to suggest. I’ve been to a few Society for Creative Anachronism events where I didn’t know anyone else there, and in those situations, volunteering to help cook/carry/organize/whatever almost always goes over well and helps you make new friends. (Plus you have a good chance of learning more about the organization, if it’s something you might be interested in doing more with in the future.)
Of course, at a big event like this, some of the organizational stuff like that might be already taken care of…but simply introducing yourself with, “I’m new, can I help?” is a nice gesture.