It will definitely not happen immediately as Haye has a mandatory defence against John Ruiz lined up, and Vitali will be fighting Kevin Johnson next month. I would guess that the time it takes Haye to fight Ruiz, taking into account the fact that his hand may be broken at the moment, would allow Vitali to fit in another defence if he wanted to. After that, as long as both men keep winning as expected, a match would surely be on the cards.
Honestly, though, I agree that he would be an underdog against both Klitschko brothers. I believe that a fight between Haye and Wlad would be 60/40 in Wlad's favour, whereas Vitali would be a much, much tougher task than many people suggest. He may not quite be on the same level as Lewis, Holyfield and Tyson during the 90s, but Vitali is possibly the most underrated boxer in the world right now. Both he and Wlad are usually tarred with the same brush as the sleep-inducing Eastern European heavyweights that make up most of the heavyweight division, but both would probably be megastars if they were American.