Although I should add, I think I've lost track of how it is being argued damage would be assigned (in theory) to a destroyed ship in the first place, so I'll have to go back and read through this all again in one go.
I just think that there are some parallels between the two examples.
I will buy the argument that the reason Vader can't take effect is that your ship has 0 health, and can't pay the 'cost' of Vader's ability, where Gunner has no such cost. But I do think that brings us right back to wanting a concrete clarification that Vader can work if you have ONE hull left, but not NO hull left.
The problem is that while I think there's actually some agreement on how some of this should work, the contradicting rulings and precedents make it extremely murky WHY some things work the way they do. If you argue that Vader can use his ability if you have 1 hull, you're arguing against the idea that the ship is removed the minute it hits zero. The ship has to stick around long enough for the damage to resolve and Vader's ability to take effect.
But then if that's the case, why doesn't it stick around long enough to take effect during a simultaneous fire situation? It's in essence saying that if you have enough damage to be destroyed, you can shoot back - and even take a SECOND shot with Gunner if you miss - but NOT stick around and use a triggered ability like Vader.
If that's the way it works, that's the way it works, but I don't get WHY it works that way and it would be nice if they cleaned up the timing issues or gave a reason along with some of the rulings.