Jak and the replica met each other back when Naminé turned into him, so while Jak wouldn't be too eager to fight and try diplomacy first, he would resort to violence if pushed far enough. And considering that he has both Light and Dark powers, the replica would probably be interested in fighting him too.

Vergil on the other hand... he would fight at the drop of a hat. But I honestly think it's safe to say he's as strong as Sephiroth, so that might end badly for poor little Repliku.