The effect from Marksmanship says "this round" and "when attacking". Does Dengar's "payback" attack get the benefit from Marksmanship as well as his original attack?

Yup! In fact, marksmanship is typically recommended only when you know you'll have the opportunity to line up multiple attacks in a round.

I'm thinking of flying Vessery, with cannon title when it comes out. Two fully modified shots, I think it'll be fun lol