I have not heard of a permission problem in this area, but you could check vBulletins JIRA as a sanity check.
As @forceHss said you need to create a plan to review and test your permissions.
To check a users current forum permissions: Under the
*User Manager in the ACP under "Quick User Links" --> "View Forum Permissions".
*
User Manager - ACP --> Users --> Search for User -- Is what it should have been called.