hi when you get 100% you get 200 health, 200 armour , a t shirt that say "i completed vice city and i i got was this lousey t shirt" or something along those lines, also all vehicles now have double damage. i think thats about it ..
if you go to the Grand theft auto forum found here you will get all the information that you could possibly need to complete the game..
Or failing that you could also try here this has walk throughs guides to the locations of hidden packages etc...
