What's Vayne's connection to Warwick, Vlad, and Runeterra?

Warwick's connections to Singed is obvious, and I can see him being a foil to Camille, but why is he associated with Vayne? I get that she's a hunter of dark magics, but the guy's a chemtech experiment gone wrong (or right). She's also a Demacian, and I don't think they have free access to Piltover and Zaun, while Warwick doesn't seem interested in areas outside of Zaun.

Speaking of Vayne, she seems interested in Vlad. She's a hunter of dark magics, but if her old lore is still canon, shouldn't she be more concerned with witches? How often does she travel the world?