I would normally lay the pedal crank arm horizontally in the vice, giving more clamping force on the arm, but to make it easier to view both sides of the freewheel assembly, i set things up vertically.
It wouldn't take long for a person to see that the vertical method won't sufficiently hold the pedal crank arm in place, and just slip out of the vice; attached to a home workbench.
I used a crescent wrench (known as a "shifter" in Australia) because most people wouldn't have a box end wrench (known as a "ring spanner" in Australia) of that size in their tool kit.
I agree that a proper shop jig would make things easier.