In addition to TMF, I belong to a few BDSM sites . We use the term "Vanilla" ,on those sites quite a bit. To them, at least, the term means anyone who is not into any of the bondage, or other lifestyle, of any kind, and just someone who likes "normal" (Whatever that is) sex or foreplay, without tying up, tickling, spanking, or other such activities. Many people on those sites have partners who are not into the bondage lifestyle, so they come to the site to chat with like minded people.
Hope thats a good explanation.
Mitch