Does it become a critical hit as part of your attack that the opponent can evade with defense dice or is the critical hit automatically inflicted on the opponent's ship?

The answer is on the card.

You are instructed to suffer 2 damage in order to cause the enemy ship 1 critical damage AFTER you perform an attack on it.

It can't be part of the attack sequence if you are instructed to do it after the attack sequence.

Not a **** thing you can do about it

That's why Vader is scary for ships like fel.

So long as he has a shot, after the attack he can take two dmg to his ship to give a crit which is unavoidable

A Decimator with Vader and gunner puts the fear of God into interceptors

It can not be evaded but it will first take out a Shield Token if any are remaining.