A word used to describe someone with little inclination to do anything spontaneous or exciting, either sexually, or in general every day life. A vanilla person will not be open to try new and exciting things, instead prefferring to remain dull, innocent and safe.
BOY BITES GIRL ON NECK
GIRL: What are you doing?
BOY: Biting your neck - d'ya like it?
GIRL: Eew! Get off!
You're so fucking vanilla, Toni.
"You can't take me into the heavy metal section! I'm way too vanilla for that."
"Of COURSE Hannah's still a virgin! Look at her, she's SO vanilla!"