You can always get a premade vampire costume, but if you want to put one together yourself, it is really very simple. You can do it and really only have to buy one thing: vampire teeth.
There are a few cool tutorials that show how you can do sexy vampire makeup.
What I would suggest is to figure out the outfit. A cool sexy gothic look would be jeans with a corset style top. Then do some vampy makeup and add the teeth and maybe some bitemarks.
Here are some awesome accessories to check out.
Vampire Teeth:
I like the fact that these teeth get really good reviews. If you have every worn the cheap plastic kind, you know that they look really lame and dont ever fit right. These are a good pick if you want something that looks hot and scary.
So once you have the teeth, then pair them with a sexy outfit. Black pants and a dark shirt would look cool. Or a tight black dress. Anything that has a cool dark and edgy look to it.
If you want to splurge, then this would make an awesome outfit. Tell me this outfit woudl not look amazing. I like this corset top because it is a top of the line piece of clothing. You can wear it out on other occasions. The main thing women should if they can is buying a piece of clothing that will only (or can only) be worn once. This corset is an awesome top that can be worn to clubs, parties, and lots of other non-scary events!