Instead of the vise clamping that fitting even tighter, maybe two box-end wrenches to put the stress equally on all six points - ?
Or, a socket or box end wrench mounted in the vise then the fitting slipped into that. I think the clamping action of the vise is what is making this impossible.
Then there's always heat. As my second paragraph but with the female part of the joint heated from a propane torch to expand it. Maybe chill the male part with an ice cube.