Assuming you're talking about the coolant vent located on the thermostat housing, then take a proper sized wrench and hold the tube with that while using another proper sized wrench on the plug. Maybe try a little bit (and I mean a LITTLE bit!) of heat. I don't recommend vice-grips for much as they destroy the finish and can wreck the parts pretty easily.
Clean up the plug on a bench grinder with a wire wheel as it's probably built up a bunch of cr@p on the threads, and the tube using a small brush.