Samsung Vibrant WiFi Tether

I don't think TMO offers it. But you can always root your phone and install one of several wifi-hotspot apps out in the wild.
 
tethering

You can tether with T Mobile. I did it with my G1 using PDAnet. You just have to make sure PDAnet is installed on the pc/laptop/netbook as well as on the phone and you have to turn on debugging mode on the phone. Works great for me although I haven't yet tried it on the Vibrant.