Vanilla is slang terminology used in the BDSM crowd for couples that enjoy a relationship free of any sexual kink. It does not necessarily imply a missionary-position-with-the lights-off-only type of pairing, but rather a relationship where there is no declaration of who is dominant over the other, where there are no ropes or handcuffs involved, and pain is not used or seen as pleasurable by either party.
In other words, if one has what they consider to be a "normal" relationship, then one could define that relationship as vanilla. Though almost all BDSM participants feel that what they do is normal and natural, they feel that they have relationships with more "flavor," and therefore do not classify themselves as "vanilla."