I work at Superhuman. We used to have code to detect Vimium, though we evidently we removed it very recently, while working on something tangentially related. It was working but definitely finicky over the years.
The initial way we checked for it was looking for document.body.matches('*[_vimium-has-onclick-listener]') but apparently that stopped working in 2018. We then used a variation of this code: https://github.com/hackape/Detect-Vimium/blob/master/detect-... which seems to work still.
How long ago was this? There used to be a way to do this for earlier versions of Vimium but that doesn't work anymore. Very interested in knowing if it's still possible. Will check out superhuman.