I'm on the emacs side of things, but knowing at least the bare minimum of vim is handy, because I have run into into systems (usually very small systems like routers or something) where some vi variant is available and nothing else is. Though as systems get bigger, it has become more the norm to have at least nano also available.

I'd know at least this:

• i to enter insert mode. Then you can edit as in a non-modal editor.

• Esc to exit insert mode and go back to normal mode.

• h, j, k, l move left, down, up, and right. The fingers under your right hand on a QWERTY keyboard.

• / to start a regex search

• % and then SRC/REPLACEMENT to do a regex replacement.

• :q to exit without saving changes.

• :wq to save and exit.

That's enough to perform a couple of small edits or something if need be.