Yes, it is best to drill off of the engine.
I would recommend, getting a bushing that the plug fitsinto from the hardware store, then installing it in the bushing, so you can clamp it tight in the vise without damaging the threads on the pipe plug. Who cares if you damage the threads on the bushing, they're cheap and that way you won't damage the pipe plug threads with the vise jaws while clamping it tightly so it does not move while drilling.
Yes, that's one of my favorite Monty Python skits. When my mom and grandma start bickering/arguing, I just refer to that and say, "I came here for an argument..." And they look at me funny...
