A-Trac is used in really dicey situations, or when you're stuck or likey to be stuck. It works by sending power to the wheel with the least slip while at the same time braking the wheel(s) with the most slip. Only works in 4Lo in part time 4WD SR5.
VSC is Vehicle Stability Control. It works in a similar sort of way - but it stays on all the time unless you shut if off. It basically saves your butt when the truck starts to slide sideways, in either an oversteer or understeer situation by braking certain wheels, depending on direction and yaw rates etc, to keep the truck going straight.
If you want to burn doughnuts in the snow, shut VSC off. It can be shut off by holding the traction control button (not A-Trac) for a few seconds. You will see the slippery car tracks icon light up to tell you it is off.
So, A-TRAC is for slogging through the tough stuff, VSC is to keep you from ending up in the tough stuff.
Hope that helps.