"What do you think is important in keeping a BDSM relationship healthy? How does it differ from a vanilla relationship?"
I don't think what's important for a BDSM relationship to stay healthy is all that different from what a so-called "vanilla" relationship needs to stay healthy. You need trust, and understanding, and constant open communication. You need to feel like you have a partner who understands you, and even if s/he doesn't always understand you, s/he will respect you enough to accept what you're trying to convey. The only thing different in a BDSM relationship is that the needs of the individuals are probably very different from what you would find within a "vanilla" relationship.
On the other hand...how the hell would I know what goes on in a vanilla relationship? I mean, what really goes on behind the scenes? Sometimes I hear things on TV or on the radio, things that I suppose other people take as face-value truth, and I think to myself, what the hell?
Like, last week I was listening to the radio and the hosts of the show were asking listeners to call in and answer a question: if it meant you would lose 30 pounds and keep it off for the rest of your life, would you agree to have sex with your partner everyday for the rest of your life?
All I could think to myself was, why wouldn't I agree to have sex with Husband everyday for the rest of my life? Why would I have to be bribed? Why is this such a big deal?
Which brought up a whole laundry list of other questions: do "vanilla" people not have sex that often? Is it a chore? Do they refuse their spouse/significant other? Do they use sex as a weapon in the relationship? Do they expect some kind of favors for sex? What the hell??
So you see, I can't really assume much about "vanilla" relationships, because I just don't know. But then again, who can really know what goes on behind closed doors of any relationship? Some BDSM relationships are a complete mystery to me, too.