All we know is that Mihawk is slightly better than Shanks at Swordsmanship but that is pretty much it as Oda has not shown Mihawk being able to use Conqueror's Haki. Honestly we really can't say how strong or weak he is. All we know is that he is the best swordsman.
P.S. Don't underestimate Vista! He is a highly skilled New World pirate and one of Whitebeard's top fighters so Mihawk shouldn't have owned him in the first place. Of course Mihawk would of one but Vista is no plus over!
Mihawk is the worlds strongest swordsman capable of pwning Zoro with knife the size of your finger,, so yeah, he's pretty strong, I mean, he cut a battle ship in half after Nami left for cocoyo village! But remember Vista was one of Whitebeards commanders in the new world, so he's obviosly no piece of cake either.
The fact that both Shanks and Whitebeard are yonkou, does NOT mean that they are on equal footing. Whitebeard is a lot stronger than Shanks. If they fought, shanks would lose. Mihawk is on equal footing with Shanks (at least he used to be before Shanks lost his arm. It's unknown if they are still on equal footing, because Mihawk refuses to fight a "one-armed-has-been".) But, he is not on equal footing with Whitbeard.
Mihawk is pretty strong. We know he's still a lot stronger than Zoro and we know that he is probably a bit stronger than Vista - Probably not by as much you'd think, but by enough that, after a massive fight, Mihawk would win, but not by much. Against Shanks, however, he would lose, I think. I have no idea if Mihawk can use Haki - I imagine he would, but I haven't seen anything to confirm it. Even if he could, Shanks has powerful Haki, that would likely even rival that of Rayleigh.