Personal Vault shouldn't be limited
The 3-file limit on the newly-introduced Personal Vault should be removed.
Show good faith to your free users by removing this limit. No-one is going to pay for OneDrive for this option. Instead, you should protect the files of your users around the world (by means of the ransomware protection) without charging for this functionality.
Do this for free, with no limitations, and you will garner loyal users. Making this a pay-only option (for more than 3 files) will push users into the Hate Microsoft camp.