I actually have the opposite opinion.
Shyvana's stance in the splash just seems wrong for her. She never walks like that in game, she always has a wide stance. Wide stances grant stability and power because you're well balanced - but her seductive walk doesn't suit her personality or do her playstyle.
As for Vayne's it seems more like to me that it's supposed to look like a panel from a comic. Vayne's character takes a lot of inspiration from Batman and the stark contrasts of that picture just kind of emulate the aesthetic a "dark" graphic novel. And her outfit is black, blue and red so it's not like they added any colors for that type of 3D effect - that's just what she is!