When applied to a thing or act, "vanilla" means simply that it is "plain" or "normal". For example: "Before I found out that Sherry was a kink, our sex was very vanilla."
When applied to a person, "vanilla" usually carries both the literal meaning that he or she isn't into the scene and the connotation that this person is therefore boring and uninteresting. E.g. "That guy's so cute. It's a shame he's so vanilla."
The metaphor in both of these instances is that of the flavor vanilla being the default for most sweets in the United States.