That could work out. I would give that a run like that, then possibly another run using Vader + Concussion + Marksmanship, Backstabber, APx4.
I would open up with the target lock on the biggest threat, then the other action on marksmanship. This would give the biggest bang for that attack. Plus marksmanship could benefit Vader a bit more then other pilots as he has two actions to spend. Let me give you the math to back that up.
On the Attack Die:
You have a 4 of the sides are hits (3 normal, 1 critical), then 2 sides are blank, and 2 are focus.
That means there is a 50% chance to hit at any given time. 37.5% of the time it will be normal, 12.5% it will be a cit. Both the blank and focus are each 25% chance.
With the Concussion Missiles and Marksmanship one of the four dice is a 100% gaurantee to hit. The other three dice are then also at a 75% chance to hit with a 25% chance per die to turn one foucs into either a crit or a normal hit.
Compared to the Defense Die:
3 of the sides are eveade (37.5%), 2 sides are Focus (25%), and 3 sides are blank (37.5%). So if the target has a focus there is a 62.5% chance per die to evade. So statistacally it looks favorable to try using marksmanship along with the concussion missle on Vader.
Hope that helps you out some.
OK, assume I'm facing a Rebel squad, where each fighter has shields. I plan on using the Missiles first combat round. Vader fires first on my side, so I'm shooting against fully-shielded ships. Crits won't matter much at all. I also plan on Vader being a target, so I'm hesitant to put another 3 points on him when I don't expect him to last long enough to make it worth it. Maybe you're right and he will last long enough to make marksmanship worth having. I dunno. I'll consider it, though.