Playing Hannibal will be a good test of Vin Diesel's acting or rather the way he wants to develop his career. At the moment, he seems to have created a certain persona and played off this in most of his movies. Perhaps like a Schwarzenegger or a Stallone, you pretty much know what you'll get when he's cast. Like Kraxis, I suspect he has the acting talent to play a more nuanced Hannibal, the question is whether he will choose to do so. If he makes the role just another Vin Diesel hardass, I'll be disappointed but I'll wait for the movies before judging.
Still, we would have been on safer ground with Denzil Washington, other actor I heard discussed for the role of Hannibal. He has similar charisma to Vin Diesel but a more proven dramatic range.