On why I think Vader would win, he's one of the very best from an era known for producing very good lightsaber users (most of whom he's fought). Sure, Exar's the best of his time... but his time is known for it's fancy force powers, with multiple of the most famous masters of the time preferring to rely heavily on the force (Of Thon, Arca Jeth, and Vodo, the three big names of the teacher, only Vodo spent a lot of time advocating dueling skills. He, unsurprisingly, produced Exar and Sylvar, two of the better duelists around). So I wouldn't expect the saber level to be quite as competitive.
Unlike the force arena, where the early KotoR era is very competitive. Exar's force power is strong and varied, but Vader's got energy absorption, tk, and general good force defense. Exar having more offense and a wider range of skills probably isn't going to be telling, especially as Exar's still a saber user at heart who settles his big conflicts- vs Vodo and vs Ulic, with duels.
Or in other words, the match plays to Vader's strength a bit more. Sabers is more likely to decide it, and that's where Vader has the edge.