It will happen, if it doesn't it will be due to Wlad injuring himself again, he's old and injury prone now. But barring an injury I can't see why it wouldn't happen. Money is going to be huge for this fight, bigger than Froch/Groves if Eddie is to be believed. This could top 1 million buys if it does surpass Froch/Groves 2. That kind of money doesn't happen to often now at heavyweight. Wlad as champion has basically had 2 super fights during his career the fights with Haye and Povetkin were by far his most lucrative fights and this will be the 3rd in a long career, nobody is going to mess this up with this much money at stake.
Possibly injury. Very possible. The old man is an old man. Otherwise I think it will go ahead. I'd give it 50-50 chance of happening April 29th, as of now. If it's still scheduled a week to 10 days out, I'd say it's 99% on. These two fighters are way too professional to take drugs tests they can't pass. Both will be juiced to the eyeballs though, don't worry about that.