Vimeo, for me, is for saying "Vimeo can't play on your setup" because it doesn't recognise the user agent or somesuch, so doesn't even try.

I once heard the two characterised as "youtube will always start to play, but the quality can stutter, whereas Vimeo is smooth as butter... if you can actually get it started in the first place". Even on machines for which Vimeo doesn't exhibit my problem, it doesn't always start for some reason...