Editing Access Masks 
Warning:
Access Masks are not valid in vBulletin 5. They are maintained to show permissions from older versions
Path to Access Mask Manager: Users > Access Masks

Although membership in multiple groups is a good way to create exceptions to rules set up by usergroup permissions, they might be too powerful for some situations. For cases where you simply want to give a specific user access to a forum that he or she wouldn’t normally have access to, an alternative solution is access masks, which override all forum-level usergroup permissions.
Warning:
Access mask settings have no effect unless you have enabled access masks in vBulletin Options.
Note:
You must click Save for changes to take effect.
To edit the access masks assigned to a specific user, go to Users > Search for Users. Then search for the user you want to edit and click Edit Forum Permissions or Edit Access Masks.

To view existing access masks, go to Users > Access Masks.

Access masks work similarly to forum-level usergroup permissions: inheritance to child forums still occurs. However, you do not have as many options as with a forum permission:
User Contributed Notes: Editing Access Masks