Hmm, yes. Form what I’ve found it should be working, but it just seems to keep the video’s aspect ratio and full width. And not fill the screen like the Youtube embed.
But I have found this: http://dfcb.github.io/BigVideo.js/
Which works on the tag but will also take vimeo direct upload links. SO I think I’ll use this for this project. I hope it helps anyone else as well.