The title is the World Strongest Swordsman not the World most Skilled swordsman.
If Oden and Mihawk fought and Oden won because of his adv CoC would you say Oden is not the WSS?
Ohh, I see you misunderstood your logic or misinterpreted me.
If VISTA gets KIZARU's DF, and he still considers himself a swordsman and Mihawk defeats him, he is certainly a more powerful swordsman than VISTA...
So, even if VISTA uses the DF, as long as it considers itself a swordsman, the Mihawk will be superior.
But little grasshopper, what if VISTA doesn't consider himself a swordsman?
This will be a fight like Mihawk against Akainu, Aokiji against Mihawk, as Mihawk's opponent is not a swordsman, and in that sense it will be a fight of different combat classes even if your opponent uses a sword like KING, like Big Mom, like Kizaru against Rayleigh.
Mihawk = Best in sword class.
Big Mom = Uses swords, DF, punches etc ...
It is the duty of the swordsman class to combine with other combat versatility, the mihawk may be better than Big Mom using swordplay, he has the most powerful swordplay in the world, but that doesn't mean he is more powerful individually than Big Mom using 100% of his arsenal of powers not only hold himself (by restricting the swordplay itself), but that also doesn't mean that Mihawk, as a swordsman, can't match
with Big Mom using everything he has, he needs to be able to do that or else his fighting class is quite limited, and I think that limitation does occur, especially for certain people in this manga ;).