Given the point cost difference and what Pilot skill can mean on a turret, I'd run the PS8 with Vet Insticts most of the time, just to know I can eat Phantoms alive. Vader doesn't do that nearly so well.If I were to use him (because quite frankly, Vader is not really my kind of upgrade) I would put Vader on the lowest PS VT-49, not the highest. I would also not use him on every shot, just when it counts. But, it could be useful to land a finishing blow or an unwanted critical. With the turret, it should be easy to have a shot on the sick prey trying to flee.
But Vader is not my personal cup of tea. I don't use him in a Shuttle and I don't think I will use him in a Decimator. I do believe it can be nasty though, especially against Interceptor or any other high agility/low hull ship.
I like the Vader card, it has a lot of uses, but the shuttle is the best place to use it. For 1/4th of your team you get a lot of danger and drawn fire(akin to Biggs but with twice as many hits.) For 40% of your team you already were drawing all the fire, losing all the tactical advantage.
I personally think Vader will be an OK upgrade, it is just that there are so many better ones available out there. Unless the Decimator comes with a way to regen shields/hull.