Hi Andrew,
I can understand the frustration caused by the issue. I think this is some ongoing issue with Vault on Web, as several reports are seen by me lately:
https://learn.microsoft.com/en-in/answers/questions/5938057/cant-access-my-personal-vault
Please open OneDrive on the web first:
https://onedrive.live.com/
Use this link to get in touch with the OneDrive support team:
https://go.microsoft.com/fwlink/p/?LinkId=528676
A OneDrive agent should assist you further and notify you when it is fixed.
A friendly reminder that this is a community support forum where users help other users. Advisors here cannot make changes to Microsoft services or access internal systems. We can only provide technical guidance, suggest troubleshooting steps based on your issue - have a great day!
I look forward to hearing from you to get the results of the troubleshooting.