Embedding Vimeo used to work, so something has changed; either with Vimeo or with the forum software. I might ask over at the SMF forums if anyone knows but if Bill one day gets around to updating this forum to the latest forum software embedding Vimeo isn't supported in that so it won't be of much consequence anyway.
Edit: It appears Vimeo embedding was a mod to the standard SMF install so my guess is that something changed at Vimeo's end. Still, if Bill upgrades the forum embedding Vimeo isn't part of the standard install anyway and the mod may not work in the latest forum software either.
- Tim