I've threaded pipe hundreds of times.
Go to an auction of a closing electrical/plumbing contractor and get
yourself an American-made trivise made for the deed. Don't worry how
crappy and old it looks.
The seat for the pipe has got to have teeth on it and then chain that
you can then really crank tight.

Your vise can't hold 1/2"....hahahaha ! (not laughing at you)

I've threaded 2" ridgid conduit many times with these trivises.
One usually needs to use a cheater bar out the end of the hand
threader...and it helps to simply lay the weight of one's whole body
across the bar to offset the torque.

You have to tighten the vise as hard as you can but it, amazingly,
will hold even against those kinds of torques.