With these options you can control visitor messages on users profiles.
vBulletin Options > vBulletin Options > User Profile: Visitor Messaging Options
Maximum Characters Per Visitor Message
Maximum characters to allow in a visitor message. Set this to 0 for no limit.
Default Visitor Messages Per-Page
This setting allows you to define the default number of messages displayed per-page in the user profiles.
Maximum Visitor Messages Per-Page
This setting allows you to limit the number of messages users may display per page in the user profiles.
Visitor Message Moderation
When enabled, all new visitor messages are placed into moderation. This can also be enabled in usergroup permissions.
Allowed BB Code Tags in Visitor Messages
This setting allows you to enable and disable the use of various BB codes in visitor messages.