I recently won a cultural with Alexander as my friend - the physical layout was such that he was my entire landside border, and after holding him off in one early war, got a substantial tech lead. By continually bribing him with techs, I kept him strong enough to fend off enemies and friendly enough that my still-substantial military was a successful deterent to his turning around. Late in the game pulled two of his cities into my cultural expansion, and even then he didn't seem to mind. Nearest I've had to a non-vassal vassal. I'm learning that giving into extortion is sometimes a good move, if there are others for a bully to refocus upon. And bribes are a great offensive tool.