Certainly for Joe Satriani, legato is a vital technique. Harmonisation and advanced, exotic scales feature heavily in both their styles. Keep and eye out for Steve's "10 hour workout" on look at getting Joe's "Guitar Secrets," a book of collected articles from Guitar by Joe. Also look on Vai's site (www.vai.com - original, huh?). There may be something that can help you there.
Hope I've helped, but I'll end with this one thing though. In the immortal words of Vai himself:-

"Don't try to play like me, try to play BETTER than me"

Think about that one...