I don't like welding on barrels, period. Unless you're way off, you can simply do what northmn suggested, make your own plug and drill the vent hole at the appropriate location. If you are way off, like 1/8", bevel the edges of your current hole a tiny bit to clear the tap marks, screw in a tight fitting blank plug made from mild steel threaded just slightly short. When it's as tight as you can get it - put your vise grips on the stub - cut it off about .030" long and peen the snot out of the edges. When you dress it off it will be invisible. Then drill your new hole. Plugging the old hole to move it is only going to work if you're moving the hole by more than two thread depths, preferably more.
Tom