I think Vinnie is underrated if anything. When he needs to restrain he does (eg. in Kiss and on "All Systems Go") and he is a brilliant songwriter as well. But I think as far as the speed/shred playing goes, the difference between Malmsteen & Vinnie is that with Malmsteen it seems like the speed isn't there for a reason. The songs are melodic, the production is very polished and the guitar sound is very crystal clear... yet he's playing a million notes and it really just seems out of place alot of the time. With Vinnie Vincent, in the case mainly here of the VVI debut, the songs are over the top, dripping with attitude, intense & ear piercing and Vinnie's guitar tone is really a "screaming one" that shatters your ears at high volume, so the million notes a second in that situation has more of a purpose as it adds to them intensity of the songs, and more melodic, slower solos might take away from the overall energy. Whereas with Malmsteen I think more melodic AOR sounding solos would add alot to the songs.
Malmsteen & Vinnie are both excellent & underrated songwriters though. But this is a totally separate issue to their guitar skills. As a guitarist, Malmsteen is far more highly regarded than Vinnie Vincent and I don't think he deserves to be which is why I think Yngwie is a bit overrated and Vinnie is certainly underrated. Especially when you take into account that speaking purely technically in terms of speed & difficulty (not that its important though), Vinnie wins out over Malmsteen by a long shot.