The vaulting of certain characters is a stopgap solution to the dilution of the 3* pool, and can only work so long before it causes as many problems as it solves.
Possible solution - allow us to pick X number of people essentially to be our own personal “Vaulted” list. Every time a new character is added to the pool, we can add a 3* to the list. This would be a sustainable, long-term solution to the pool dilution, and, while it would require some new coding, a) I don’t think it would be too difficult, and b) it would also solve this problem long-term. The only real problem would be event tokens where someone you have vaulted was on the boosted chances list, but I think that could be worked around easily enough one way or another.