Do you just count it as an additional wound? If so, does it ignore soak?

By core rules,

Against henchmen (the shared wounds groups of mooks) crits just convert to more wounds.

Against individual NPC's/monsters, crits apply as they do against PC's. If damage is inflicted other than by "minimum damage" rule, and crit indicated then a random crit applies. Crits tend to hurt monsters a bit less as many are more "longer term oriented" or target actions monsters may not be taking, whereas others still "ouch" and have players cheering.

Rob