Your phone should transition automatically, simply the extra signal/space for the 3g would not be there, the 3g parts should drop, the 2/2.5G should always be there. Your phone would have a 3G reciever and a 2G reciever, i would say this is more an issue of the Vodafone network than your handset, on mine, K800i and prior K608i when i had no 3G coverage the 3G symbol just disappeared, the 2G component of your handset should still and always be connected. I can't comment on Vodafone as i'm a three subscriber and count myself lucky never to have had any of the issues a lot of people here did,