Bagha Khan has an EXTREMELY HIGH dodge rate once he reaches max and you train all the way to Dodge 4. Giving him Riposte, especially Riposte 2 or 3, would make him OP, so instead, KI gave him Vengeance Strike.
Why can Bagha Khan train Vengeance Strike instead of Riposte when he's a swashbuckler? I'm a bit aggravated by this.
It's just the say he is, Dead Mike is a buccaneer and can train riposte, but not vengeance strike. Milo Greytail can train vengeance strike, but not riposte. It's all how the companions are made, I guess. Hope I helped!
I suspect it is to insure a bit of variety. Sometimes Swashbucklers have access to skills normally associated with Buccaneers, sometimes Buccaneers have access to skills normally associated with Swashbucklers, etc. I think it would be a bit boring if all companions of a given class had exactly the same skill mix.