The full Gmail site loads fine and always has. That said, it's quite slow and a bit of a pain to use (unless you like using the stylus) so I've never done anything with it beyond opening a folder and opening an email. Can't comment on the other functionalities.

Vimeo videos have never worked on the N900. The video generally doesn't show up at all (and when it does, you get one frame every 5 seconds) and audio is very choppy.

In general, support for Flash videos on the N900 is very much a hit and miss affair with, in my experience, quite a few more misses than hits.

Of course, there's a bug for that but it's been closed and marked as WONTFIX in Maemo 5