Scenario:

I have got Vader Commander and a Devastator with Gunnery Team.

First attack, i use Vader to spend a defense token for a reroll.

Then the second attack from the front arc.

Can i first roll attack, use Vader for a reroll (discarding the spent token) and THEN use Devastator for an additional die?

I believe you can because it is still while attacking.

Correct, you can, It is still during your "while attacking" step.