So Rex fire supports in a melee against Vader

Vader has a dodge and rolls a few surges

Rex support is ranged so the saved attacks are deflected back?

If so, Does Rex take the wounds?

Can the attacker assign the wounds to the melee unit?

Fire support just adds dice to an attack. It does not change the attack to ranged and it does not make Rex the attacker. So no: Vader cannot deflect and if he could (like Obi-Wan can), Rex cannot take wounds from deflect, all deflected hits are dealt to the original attacker.