What are your current export settings? I've exported to Pro Res before uploading to YouTube in the past, but I'm not sure Vimeo allows that. I probably makes a minimal difference anyway. Adobe Media Encoder has H.264 presets for Vimeo that I typically use now. Bumping up the bitrate may help, but unfortunately Vimeo is going to re-compress whatever you upload anyway so it may not solve the issue.