Eh, I think Cage can pull it off. Maybe it's just because I'm not a hard-core fan, but I've never really thought the Ghostrider character was defined enough that the movie-version needs to be exact to do it justice (unlike, say, John Constantine). I think, if anything is going to kill that movie, it won't be Nic Cage as Johnny Blaze, it'll be a flaming CG skull.
And yeah, I agree about Vinnie Jones, I just can't see it. But I guess he's a better choice than any of the typical wrestler-turned-actors (with the exception of the Rock, who I think has become quite a good action hero in this new Arnold-less Hollywood). But let's face it, Juggernaut has one of the most ridiculous looking costumes in the comic books - and that's saying something - so just about anyone will look funny in the suit unless they pull off a phenominal new design job on it.