Usually. But it of course depends on the couple. I say that my relationship with my partners is generally vanilla since no one is collared or owned or submissive or dominant to another on a day to day basis.
With my wife and I it is a matter of the marriage coming first, you may call it vanilla in that sense, but we also enjoy playing together with submissives. We do enjoy polyamorous relationships that have BDSM and D/s included in them, but even with these the relationship comes first.