I am not sure how Wladimir would do but Vitaly would have been quite succesfull, considering he was 6 times world champion in kick-boxing and his superior reflexes.
I don't think he'd do all THAT well. He could definately be a top-notch striker, perhaps the best in the MMA HW scene, but his body is just not built right to be a top-notch wrestler and/or submission artist. He'd be very weak to certain types of attacks. He could be a bigger version of Cro Cop, though. I don't exactly agree that all boxers could necessarily be successful if they trained for MMA instead, and vice versa. There's a big difference between a wrestler and a boxer/striker at the lowest level. Granted, most of these guys are athletic enough that they could definately be contenders if they had gone into a different sport, but comparing boxing to MMA is like comparing American football's halfbacks to its cornerbacks. There are shared talents, and athleticism always helps, but they are different sports. MMA's a lot more forgiving when it comes to styles than boxing is, but alot of these cross-over type athletes could just end up as 1-dimensional guys.
Vitali would probably do better than Wlad provided he makes it through training. Vitali already has the world class kickboxing experience - so all he needs to add is some wrestling and submission defense.
Vitali also got KTFO by Pelee in kickboxing, so he wasn't really all that. He would still have superb MMA potential though.