I really wouldn't worry about it too much unless you're mixing all six schools together. If you are then yes, it could make the Legendary Spells a little bit more rare and means that not every Legendary spell will be in play. On the other hand, I've yet to see more than two Legendary Spells out in a game.
I wouldn't be too surprised if Trey decides to add an extra scroll if they release another expansion.
i wouldn't worry about adding another scroll. I have seen 3 legendary spells in a game but never more. if an expansion adds a significant number of vault cards then they may add a scroll, but until then I don't think its necessary.
The Student Stores and the Vault were in play, and I was frequently buying two or three cheap consumables in a round because I was aiming for (and won) the most gold and most consumables votes. So we cycled through the vault deck quickly.