Would the placement of Vanille's mark have mattered as much if she were wearing shorts instead of a skirt?
Honestly, pretty much all of sexualization in Japan comes in the form of girls in skirts. It doesn't matter what they're wearing on top. Hell, they could be in a big, uncomfortably fluffy coat, or have a robot torso, or be wearing a trashcan/cardboard box, as long as we can see she's wearing a short skirt down there, she's earned the "sexy" label. Yes. I've actually seen this.
I'm not going to say it's undeserving. I'm the one who literally can't because I like girls in skirts. I'm not sure why. I really don't care what they are wearing on top (for all I know, they may even be significantly lacking in that department KWIM) but if a cute girl or sexy lady is wearing a skirt, I can't help but watch. I think it may be some childish fascination that I never got to play out. As a kid, I never met any girls who wore skirts so I never "punk'd" them with the "grab and lift." There may have been one skirt, but as a kid, I was shy and inactive so all I would do was sit and stare.
Anyway, I think that the Japanese have a thing for girls in skirts (hell, every schoolgirl has to wear one as school uniform.)