Question: Does Gun-Fu only apply to guns? The tooltip just says "does 25% more damage to your second target and beyond in vats" but doesn't actually specify that it has to be guns.
There's no explicit gun requirement attached to the perk's effect, but it also uses an entry point specific to Gun-Fu, so there may be a hardcoded (and invisible) restriction.
Easiest way to check would be to get up to the third rank and see if you get the auto-crits with a melee weapon.
Perk descriptions have no effect on gameplay mechanics. I can mod the description to say "Gives you five million health," but it still won't give you five million health unless I make changes to the perk's actual effects.