Can Vader activate twice, once after each assault missile attack? Assume there are 20 hull points on the ship Vader is on.

Is this question intended to be about Cluster Missiles?

Assuming this is about cluster missiles I see nothing which prevents you from using Vader twice.

Cluster Missiles are separate attacks, and Vader could trigger after each one.

If the question was about Assault Missiles, you could trigger Vader against the primary target of the attack, but not anything hit by splash damage, as you can only use Vader against the ship you attacked.

I love the Vader crew card!!

It essentially is "cut hole in hull with light saber to cause ship you just attacked to lose one shield"

Like he's in the crew compartment and thinks to himself "you know, if I activate this thermal detonator I could blind that A-wing pilot for a spell...or at least take one of his shields." And the co-pilot says to the pilot "I hate it when he just sits there quiet like tha...Oh SNAP, GRENA..BOOM"

A-Wing pilot is all "Hey, one of my two shields is gone!!!'