I dont think she can be a leader in the Vale for several reasons: neither her mother or father were leaders there, it's not her home, she constantly thinks and dreams of WF, it's dangerous for her there with all the lies Littlefinger forced her to use, and this 13 year old girl is not going to be able to take down - all by herself - the political mastermind who fooled Tywin and Tyrion. She will need help from her siblings, and that means she can out maneuver LF later; it doesnt have to be in the