"Vanilla" refers to any item or equipment that was available only in the game when it first came out. As each expansion was released, first The Dark Below and now House Of Wolves, the "Vanilla" gear that was sold by vendors or dropped as rewards has been changed. A lot of stuff you can tell is "Vanilla" because it cannot be upgraded, since it was around before that was a thing in the game. Also, the little triangle in the corner of your gear says which expansion it's from: if it has "I", it's TDB. If it has "II", it's HoW. Hope that helps!