I went on a compressor binge a while back and had a few. Of all three Boss models, I prefer the CS-2.
The Ibanez CP9 is nice, but I found it to change the fequency resp too much for my taste...kind of a mid hump like the TS9. Also had a couple old MXR Dynacomps (block logo), which are fairly squashy and milky smooth.
I still have the CS-2 and a DOD Milk Box, which is amazing for the money (Clean, clear, flexible, and cheap). I also recently bought a use Analog Man modded DynaComp (Ross mod/true bypass)...it's nice, but to me compressors are relatively subtle and I don't find huge differences.
As far as Vai, I'd suspect he does not use compression other than what his
tube amps do naturally...I don't believe I've ever seen a comp on his board, but as GP indicated, he might use the comp in a rack processor.
Greg