its haki , bs haki is an invisble shiled that means orignally it dsnt appear but if you have experince you can concentrate it in a part of your body which makes it turn black (harden) and use it as wepon wich makes the hardend part stronger plus bypassing df powers of course ( example like in the opening luffy is using bh hardening to fight akainu with fists without getting affected by his maguma )Are we sure that was haki and not just monet not activating her logia ability? Honestly no opponent has appeared yet that imo would make zoro bust out armament.
zoro didnt show hardening but oda said bh is his speciality so 90% he can do it imo otherwise oda saying that would be irrelvant . just wait tow ot three arcs and you will say zoro vs vegro ? nah its a low\negg dif (i see love cook jmping and sayin ok save your talk untill those tow arcs come )
as for now zoro didnt show an abillity to cut throw vergos haki so for now vergo wins