User blog comment:ClariS/Unofficial PBE Patch Notes (148)/@comment-218.186.15.10-20120916111222/@comment-4743916-20120919162354

The idea behind Vayne's character concept was a ridiculously squishy AD for her range in exchange for having a high skillcap due to superior mobility and focusing power. If they just give her back some of her mobility, it'll be a lot more practical to play her in teamfights.