Detect whether the Vimium Chrome extension is installed. Tested on Vimium v1.59. As for an official solution to Vimium detection, keep an eye on Vimium issue#2399
hasVimium
returns true if Vimium is detected, false otherwise. This detection is not 100% accurate.
if (hasVimium()) {
alert("Hey, you have Vimium!");
} else {
alert("I don't detect Vimium, but it might still be there.");
}