GSM is GSM (assuming that the unlock worked properly).
before you go, borrow a friend's T-Mobile card just to make sure the unlock worked.
once you get there, just buy a pre-paid sim, insert the card and enjoy!
AFAIK, no pre-paid provider in Vietnam offers GPRS (or EDGE) on their prepaid plans (so only data available to you will be WiFi)
my last trip to Vietnam, I used my factory unlocked HTC TyTN there with no problems with a local SIM card (Vinaphone, i think...)