I had asked the same question in IRC a few days ago. They said that MyBB is allowed. I believe the reason they stated that IPB and vBulletin aren't is because they are resource hogs, and I'd also assume high level of piracy.
You should be safe to use MyBB

In fact, I plan to at some point when the vndv ftp issues are fixed (has been a week now and no response whatsoever...).