I just got the game and have played a handful of games. Vader got reduced to level 0 on a crit card. So i went first in movement phase and did a evade and focus after my move. I had the card, cant think of the name , That let me reroll one of his damage die but take a stress token. My question is does that wipe out my focus and evade token or do I still get to use them? Thanks for replies in advance.

You had Elusiveness

"When Defending you may receive 1 stress token to choose 1 attack die, The attacker must reroll that die. If you have at least 1 stress token you cannot use this ability"

You assigned your tokens at the end of your Activation for that ship.

You keep them until used. Stress only means you are not be able to perform actions (or perform red maneuvers), but you can still spend tokens already assigned to your ship.

Stress generally only prevents you from taking ACTIONS or performing RED Maneuvers. It does NOT hinder your ability to use any tokens you may have or any non-Action/maneuver things you could do unless that other thing specifically calls out stress.

Even if you have Stress a ship could still be ASSIGNED or Acquire various tokens provided the ship is not performing an Action to get those things. A card like Fire Control System causes a ship to acquire a TL on the ship it has shot at; it gets to do this even if it has stress because it is NOT performing an action to acquire that TL. A card like Predator would allow a stressed ship to reroll attack dice as it isn't an action.

Thanks!