Well, you can't 're-lock' it to another carrier that I'm aware of (nor is there really any reason to). Once you've unlocked the phone, just throw another SIM card in and you should get regular service.
As far as everything else working, the only thing that comes to mind is the frequency differences between T-Mo and AT&T; the Vibrant lacks AT&T's WCDMA 850 & 1900 frequencies, so you may not get 3G on their network with that phone.