I truely believe that you are looking for an alnico pickup.
I honestly believe that the greatest sounding pickups use alnico magnets. Sure, there are a lot of good ceramic magnet pickups availible, but I would recomend an alnico humbucker to you.
It seems that you do want a fairly versitile humbucker- if you want Vai sounds AND SRV type tones.
All of the players you mentioned used high gain pickups (well, the SRV pickups are said to be hot, if not high gain).
Therefore, it seems that you are looking for a 'hot' sounding pickup (more winds- more DC resistance, I believe).
I admit that I am VERY biased, but I think that you can get a great high gain sound out of an alnico humbucker, but not a great clean-ish sound out of a ceramic (I am sure that there are exeptions, but as I said, I am VERY biased). I dont think you could get an SRV type sound with ceramic pickups, but I think Alnico may get you in the similar ball park.
Therefore, if I were you, I would look for a high DC resistance rated alnico pickup (an alnico 5 would probably be most suitable).
If possible (an if money permits)- I would have it scatterwound.
Both Dimarzio and Seymour Duncan have the criteria I mentioned for each pickup availible on their respective sites.
I hope thats a help,
good luck man,
Luke