That is a bit surprising. It looks like Vimeo’s player is thinking that the height of the mega menu is the full height of the screen and is sizing it’s video accordingly. Interestingly, YouTube’s player gets it correct when switching to the full screen.
The only way around that I came up with is to switch to Vimeo’s old embed code. When you go to copy the embed code for a video, down below the video in the pop-up there’s a series of checkmarks. Make sure none of those are checked. Then click the link for the old embed code. You can then copy that and use it for you video. When I tried that, the full screen worked as expected.
Let me know if you run into any trouble there,