That fight would likely look like a repeat of Vitali - Arreola. Even prime for prime Tua still stands to get his ears boxed off by Vitali.
Honestly, that would be a bad fight for both of them in a sense. Vitali would likely go the distance with him because Tua's head is made out of marble and Vitali would do nothing but beat up his own hands and keep Tua Frustrated and unable to reach him. It'd take something out of both guys at their age and do it to no avail.
Except he wouldn't. I see 12 rounds of Tua "trying to land his shot" while munching on jabs and straight right hands. Tua has never gotten it done on the next level.
I would expect Tua to get outboxed for long periods and would have Vitali as a big favourite but Vitali didn't look great last time and looks to be declining. While I am sure we will see Tua eat jab after jab he will always be dangerous for the full 12 rounds and won't have the fight beaten out of him like every other Vitali opponent, he is simply too durable and is used to being behind on the scorecards. I don't think Tua would win but he would be a definate step above the fighters Vitali has faced since his comeback, by virtue of the fact he is an actual threat even if it's an outside chance of a KO.