I think the Kemper has a lot of musical "feel" that makes me want to use it more than the "real thing". For the sake of curiosity, when I think of 80's Shred tone the LAST person I think of Is Vai. I've always thought his tone was mediocre at best. Could you name 3 Vai songs where you thought his tone was to strive for? Maybe I need to take another listen.
The whole of Eat 'em and smile and even though it's mixed pretty awfully to my taste, I love Whitesnake's Slip of the tongue for Vai's playing, too.
I'm also a massive fan of Megadeth's Rust in peace (though technically a 90's album), and I can much more easily get tones close to Marty's.