Vimeo Darker Than FCX

I spend a fair bit of time tweaking exposure and color in FCX. Why are my videos darker on Vimeo than in FCX or Quicktime? I'm working on some stuff that has subtle detail in the darkest shadow areas. Looks great in FCX and totally blocks up on Vimeo.

What about color space? I've been shooting with my Canons in RGB. Should I be using sRGB. Can I change this in post? With stills I'd convert anything going to web to sRGB.

Thanks for any tips.

Cheers.

A.T.