来自自己的需求，以及我在 reddit 上的回复。

  1. Unlock the vault when you are online.
  2. Open disk management tool (Press Win+R and enter ‘diskmgmt.msc’ without the quotes). You should see a partition called ‘OneDrive Personal Vault’.
  3. Assign a drive letter to OneDrive Personal Vault. See https://docs.microsoft.com/en-us/windows-server/storage/disk-management/change-a-drive-letter. I will assume that the letter is O in the following steps.
  4. Start command line prompt as administrator.
  5. Run the command ‘manage-bde -protectors -add O: -pw’ (change O to the letter assigned in Step 3). This allows unlocking the partition in the vhdx file with a password. For more options, run ‘manage-bde -protectors -add -?’ and read the output message.
  6. When you need to unlock the vault offline later, open C:\OneDriveTemp, find the vhdx file that contains the vault and unlock the partition inside with the password set in Step 5. When unlocking the vault online as usual, be sure NOT to mount the vhdx file, or you will encounter an error message when unlocking.

Important note: this method is not officially supported. Make a backup before performing these steps.