Based on a quick look through their profiles I'm leaning towards Victini winning. ExVeemon could win with an unconventional strategy, but each way I can think of Victini losing is something that Daisuke and Veemon would either never think of or never resort to, so I find it more likely that Victini outlasts them with more moves and a counter to each of Veemon's useful abilities.
Both have flight and speed is equalized, so neither is going to be able to avoid and rangespam which means that particular strategy is out for both, resulting in most likely a mid-range fight which is where Victini should have the edge. Mid-range means that it's stronger melee moves are still an option, but should Veemon try to fly outside the range of those moves Victini also has ranged moves to counter that strategy.
Overwrite would be a massive boon here assuming that ExVeemon could trigger it at will, but it being uncontrollable means relying on that as a win condition is faulty at best. Last I checked Veemon is one of the ancient species whose Overwrite is much stronger so had I been leaning toward Veemon as the winner that would have been enough to use as supporting evidence of him winning, but I don't see that happening so at best it's a slight advantage that Veemon has.
Overall, I see Victini winning this so far. Variety of moves, buffed accuracy, focus energy to increase chances of getting a critical hit, ranged and melee moves, and the fact that a lot of Victini's moves have a chance to leave a burn that will both slightly debuff and damage Veemon over time makes me think Victini has this. Veemon won't be overwhelmed by any means, but with better options I see Victini taking this.
For now put me down as a vote for Victini. I think there are ways for Veemon to win here, but they involve a lot of risk, luck, and sometimes even OOC action, so I see this being a win for the Pokemon at least 7 times out of 10, which makes it seem fairly conclusive to me.
