Irena, your example won't wash, sorry.
Vegetarian, vegan, gay and German, for that matter, all work as both nouns and adjectives, while a vampire is always a noun. The adjective would be "vampirical" or "vampiric".
I think Irena was just talking about the sound "a" because Neal said it could be swallowed before a "v", suggesting that even in sentences where it's required as an article, people might not pronounce it. So I assumed she meant "you wouldn't swallow an 'a' before 'vampire', it would sound weird".
I do like the word vampirical, though. It sounds very scientific.