On a website I am building I have a vimeo video embedded. The client needs to keep the sound on the video obviously for people that find it on vimeo. However for her website
You just need to add the muted parameter. E.g.:
muted
According to Vimeo, the background parameter is only supported for videos hosted by paid members.
background
Source: https://help.vimeo.com/hc/en-us/articles/115004485728-Autoplaying-and-looping-embedded-videos