vBulletin Manual - Access Masks: The Overriding Permission and Another Method for Creating Exceptions

While membership to multiple groups is a very handy tool for creating exceptions to the rules setup by usergroup permissions, they may be too powerful for some situations. In the cases where you simply want to give a specific user access to a forum that he or she wouldn’t normally have access to, you can use access masks.

Access masks will override any forum-level usergroup permissions for this user.

To edit access masks for a specific user, go to Users > Search for Users > searching for the user you want to edit > clicking [Edit Forum Permissions] or [Edit Access Masks]. You can quickly view existing access masks by going to Users > Access Masks.

Access masks work similarly to forum-level usergroup permissions; inheritance to child forums still occurs. However, you will not have as many options as with a forum permission:

Setting a user’s access mask to yes for a forum gives them the same permissions in that forum as they have at the global level, regardless of any forum-level usergroup permissions specified.
Setting the mask to no denies them access to the forum completely, as if each individual permission were set to no.
Default simply means no special access mask is specified; forum-level usergroup permissions are used.