[X] "…you are free to go."
Yeah, not really feeling Vista. She lacks and real driving motivation, simply wanting to be acknowledged. While that sort of character can turn out well (Waver) there are plenty of characters more likely to go further for the Grail. And I still believe we should actively pick masters based on that criteria.
Still, it seems like she will win regardless, and in that case I want to suggest Mordred as a servant. Both seek acknowledgement, and Mordred is twisted enough that she may end up leading Vista down a darker path. Even if that does not occur, the interactions between them could still be pretty interesting.
Also Mordred and Jekyll is the closest I come to shipping something, so that is a plus.