It's possible that Intuit finally addressed this issue in a recent update to quickbooks.  This problem has persisted for me across more than one version of QB/file manager, and across more than one computer with completely new Windows installs.   My workaround has been to keep the vault open just about all the time, and leave the computer password locked when I am not at the desk.

This week after updated quickbooks, it started working!  Unfortunately it lost all the client passwords that were stored in the vault.  Small price to pay.