"vanilla" is a term used to describe something plain, boring, common etc, ie nothing special. Most commonly it is used to describe "vanilla sex", which implies the sex was plain and boring. It can however be applied to anything.
It comes from the fact that "vanilla" is the standard (Plain) flavour of ice cream, which people often consider "Boring".
vanilla b sci is when you don't major but rather stay for like a general stream of science. UTS offers this in the B Sci (flexible) where you do a bit of everything for 3 years