It looks as if Vimeo is cracking down on gameplay videos, in what seems like an odd move for the service. In carving out a niche for itself in a world that is dominated by streaming giant YouTube, Vimeo has taken some interesting approaches to video hosting. You can't repost a video you found on the Internet or elsewhere, the rules regarding non-sexual nudity and nudity for artistic expression are more lax, and there is an increased focus on video ownership. Vimeo does not like gameplay videos, though.
The Vimeo Upload Guidelines definitely do say that gameplay videos are not allowed, although game developers are allowed to post progress videos, and machinima is acceptable if there is sufficient story involved. While this apparently hasn't been a problem before, it looks like Vimeo is starting to crack down. However, there is a course of action for Vimeo users with gameplay videos who don't want to migrate to YouTube: Pony up for $200 for a year of Vimeo PRO. That's where things get a little weird.