A Microsoft file hosting and synchronization service.
Personal Vault does not have its own separate PIN on Windows or OneDrive.com.
If Personal Vault is asking for a PIN on the PC, that is the Windows Hello PIN for that device. So, if you want to change it, you would have to change it via Windows Settings > Accounts > Sign in options. But that changes the PIN for your device too.
So, if you are asking for a specific unique separate PIN just for Personal Vault, that's unfortunately not possible. Sorry.