[snakes1100]

The maximum number of simultaneous connections permitted to any given MySQL user account. A value of 0 (the default) means “no limit.”

Before MySQL 5.0.3, this variable has only a global value that can be set at server startup or runtime. As of MySQL 5.0.3, it also has a read-only session value that indicates the effective simultaneous-connection limit that applies to the account associated with the current session. The session value is initialized as follows:

If the user account has a nonzero MAX_USER_CONNECTIONS resource limit, the session max_user_connections value is set to that limit.

Otherwise, the session max_user_connections value is set to the global value.

To answer your question though, the answer will of course depend on the activity of your users & what they are doing, also what hacks you have installed and what they may be doing as well.

You could try pconnect in your vbulletin config.php, this may allow you to skirt that limit set by them.