The Colonel has a very strong ability, when paired up correctly with other squad mates he provides amazingly accurate fire. For this reason when gifted with the TIE/D title it allows Colonel Vessery to use the pilot ability twice in a single round bringing the consistent hurt.
If you want raw damage output with splash damage, Ruthlessness is golden. Crack shot is great for getting some damage through, and Veteran Instincts takes him to a respectable pilot skill 8.
There are a tonne of viable builds for Colonel Vessery however, you'll need to consider what wingmen to fly alongside.