if u already have 3g sim and u have activated 3g in ur sim and after that u getting 3g signal then u can go anywhere in india where voda 3g is there .... u will automatically get 3g signal on roaming Untill u have followed the above statement..
some states don't have voda 3g signal so buy the sim from the state where already have voda 3g..
so it will be ur luck that u will get 3g signal or not in some state