Learning vi is a worthwhile investment for any developer. It's the editor you can assume will be just about anywhere, (You might have to bring it with you.) Learn vi before vim, so you don't become dependent on vim-specific enhancements.

Start here.
http://docs.freebsd.org/44doc/usd/12.vi/paper.html
Ignore the obsolete stuff about teletypes, broken character terminals, and slow modems, and it's pretty short. And it starts you on the right track: visual edit commands are all <operator><count><cursor-motion>, not a whole bunch of arbitrary things to memorize. If you go at it that way, and they tell you where ex(1) ends and vi(1) begins, vim actually makes some sense.

Among the vim|elvis enhancements, "left click" became just another cursor motion (unfortunately it doesn't like <count>) and "center click" a synonym for paste, but the language underneath stayed the same.