Hey Alec,
Did you ever figure out a solve for this? I am having the exact same problem. I tried both H264 and prores… The original exports are clean on my computer, but once uploaded to vimeo, the video starts to break right around 4 seconds, with crazy pixelation artifacts (looks like frame blending) for a few seconds, then is clean the rest of the video. It’s driving me insane, as its the same spot no matter what compression.