I loved Vai with Alcatrazz, I thought he was ok with DLR (with the latter's inherent vocal limits), his solo album with Devin Townsend (Sex & Religion) was brilliant, I even bought his newest solo release which is, well, interesting if acrobatics are your thing:
Frankly, the (too) many guitar tracks and an odd sound on Slip of the Tongue sounded plain weird and clinical to me upon its release. With Alcatrazz, Vai had co-created something sounding fresh,
with DLR he didn't distract (unlike with Whitesnake) and played some noteworthy solos, with Whitesnake it was: "What the flying f***?!" Any B league guitarist could have produced something that would have sounded more organic with Whitesnake. And while I never thought that Adrian Vandenberg is the Dutch Jimi Hendrix (I prefer George Kooymans), he would have created a more fitting sonic landscape as well.
Vai, a perceptive and reflective man, has admitted in interviews today that he went overboard on Slip of the Tongue (Sarzo had to rerecord his complete bass parts because his sound didn't gel with Vai's layered overdub approach), that he overshadowed Adrian Vandenberg too much on the subsequent tour (who took it with grace) and that he really wasn't steeped enough in blues and English blues rock to have been up to the task at the time.
But the majority of the blame must be laid on Coverdale himself. By then, his criteria for hiring Whitesnake members had become totally superficial. Whitesnake had transformed from a band where excellent musicians played notwithstanding their looks to one where you had to be poster material and flavor of the month as well as offer star quality, lasting benefits to the music coming in a distinct second.