So in a game I was defending with vader against a snow speeder.

The snow speeder can take 3 hits, Vader only dishes out 2.

After attackers and defenders were declared, but before anyone was focused to strike, I force choked Vader dealing him 1 damage, in order to trigger his ability to deal 1 damage to the speeder then attacked with vader to deal 2 more and finish it off before it could attack.

My question is, was this a legal move?

It depends on when you played Force Choke. If you played it in an action window (either before or after Vader's strike) then it is legal. Review the gameplay diagram at the back of the rulebook.

Yes, you can Force Choke your own units, and that will still trigger Vader's ability.

You can play Force Choke on your own units. You can play Force Choke on Vader. Both instances trigger his direct damage Reaction.

However, if the Force Choke kills Vader, I don't think you're getting the direct damage, as the Reaction only kicks in after the triggering condition (the Force Choke) fully resolves, which in my example would kill Vader and have his card sent to the discard pile where it does nothing.

As long as you're fully aware of the Action window diagram in the rulebook, tricks like this make for great plays. I've often Force Choked my own characters to trigger this as a work around to get rid of Vehicles with 1 Health (or with 1 Health remaining) that are camped out on the other side of the board.

It works really well as a way to bait out C-3P0 from your opponent, if you want to make sure you can get that Force Lightning or Superlaser off without that droid jumping on it. Choke your own guy out, put the damage on Threepio, unless they discard Threepio to cancel it, which would actually help you because they would discard Threepio, you lose Force Choke to the cancel, BUT your unit you were planning to kill survives, so you're better off.