The 2 damage to your own ship isn't really a "cost" - there isn't such thing in X-wing. But it is phrased as a prerequisite.
"Suffer 2 damage to inflict 1 critical damage". It's a "Do X to Y" structure. If you don't do X, you don't get Y. This is why I use the simple example of missile target locks earlier: "Spend your target lock to perform this attack." Nobody would accept that you could "perform this attack" without completing the "spend your target lock" part.
The damage from Vader isn't really anything like a Direct Hit, either. Direct Hit says it counts as two damage, but it's still one card. The only way for Vader to inflict two damage is by dealing two cards. And, finally, the "one at a time" clause is part of the generic Suffering Damage process, NOT part of combat resolution.