I never like these conversations about who 'deserves' a cover or not.
It's all totally subjective and everyone will have different opinions on why or what for.
But, for this cover it seems Meisel decided he wanted to use some hot new girls and chose accordingly.
Apart from her association with Prada, which I could care less about anyway, I would imagine that Vanessa will be a huge star in this industry very soon, and nabbing a Vogue Italia cover is a step towards that.