Mandalorians not rallying together to fight the Galaxy again? Three Words: Star Wars: Legacy
Boba Fett was raised like a Mandalorian child. What do you think Jango was doing all those years when he was raising Boba from birth. He was grooming him to be his successor. He most likely brought him up the same way Jaster had did him until his death.
You can't properly train a Mandalorian Child in the ways of Mandalore on a rainy water world planet called Kamino surrounded by millions of aliens who don't care about your traditions or heritage. Kamino didn't have the resources or facilities to properly train Boba in the ways of Mandalore. It was a world set up to grow clones.
Some would argue, that it was the birthplace of Clone Troopers, so certainly it would be a place designed for combat. However I would disagree, and say that the Clones didn't get much real combat training on Kamino. All virtual simulators and brainwashing. A incubator designed to grow living drones that simply followed orders.
Over glorified Droids. Yeah, Boba can't be considered a real Mandalorian being trained there. I would argue that Jango wasn't a Mandolorian either, nor is Pre Vizsla.
However, Pre Vizsla had the right stuff, but didn't have the backing of Mandalore and so he was a Hero without a Home. Sure the people of Mandalore had his back AFTER he shows up with Maul and an army. However before that he was branded an enemy of the state.
Being labeled a criminal and a thug, by the same people you are trying to free, has to play with your head. So I think Pre Vizsla suffered from confidence issues, and lacked the honor of a True Mandalorian. He did a lot of dirt to rise to power, and ultimately sold his Honor at times to get Death Watch back to Mandalore.
If Kenobi kills Pre Vizsla, then I would say that Pre Vizsla is the Mandalorian seen in Kreia's prophecy, as he was more of a Mandalorian then Jango ever was.