Also try doing some chromatic exercises, for instance try:

------------8-9-10-11------------7-8-9-10-
-8-9-10-11------------7-8-9-10------------

etc. alternate picking nice and slowly and move down the neck until you reach the first fret, make sure you are using fingers 1,2,3,4 and stop where you can't stretch to the 4 frets and go back up again. If the 8th fret is a problem shift up higher and keep trying, use all 6 strings, its a great warm up exercise and if you keep moving down 1 fret every week or so you'll be playing Vai licks in no time!