In modern days, remembering complex passwords even a single complex password for the vault is not so desired.

Possession factor (eg. physical key, file, etc) can't be used for master vault coz losing it is disaster about failure to access everything.

Inherence factor can't be used for master vault too coz those can be copied, or used while unconcious (eg. fingerprint while sleeping).

The only possible factor for master vault is memories, as said above: remembering complex long password is not comfortable. I guess this way below is more comfortable:
* Let user add a list of custom questions
* Join the answers to string, make hash of it to use as vault password or to make vault key
* After auth, user can set PIN for access instead of answering questions again

It's more comfortable coz user has questions to recall. The question list is set by user thus each user has different set of questions.