YouTube last time I tested was only giving 6 Mbps average for 1080P in H.264. While great for the average internet connection its pretty poor quality for certain types of video. They have likely made some changes along with the fact they encode into at least 2 formats (not including mobile) which effects users depending upon platform/browser.
YouTube forces an advertisement first if you go to YouTube. com while vimeo.com doesn't. for mobile viewing this is a significant downside to YouTube.
it's easier to get the video in full screen on the video site than on xenforo.com
I noticed that on Vimeo the resolution of that specific video is 720p, they downscale it. @Kier you have a PLUS account, so take a look here, set to 1080p and reupload it so we can view it at 1080p like the YT one.
From a potential marketing standpoint, it might make sense to do both. That way if anyone just so happens to be searching for XenForo videos they might have a better shot at finding them at their preferred video site.
I voted YouTube too, but only because there was a "no preference" option and I use YT more often than Vimeo. But, if I'm honest, I prefer text-based HYS over video ones because it's generally too noisy in my house to listen to them. I much prefer to read and look at screenshots.