A solution that I use:
You can use random mysql connections…

At vbulletin config.php instead of :
$config['MasterServer']['username'] = ‘uname’;
$config['MasterServer']['password'] = ‘pass’;

Random mysql users code (three mysql users example):
$dbusers = array(
array(’user’ => ‘mysqluser1′, ‘password’ => ‘pass1′) // First MySQL user/password combination
, array(’user’ => ‘mysqluser2′, ‘password’ => ‘pass2′) // Second MySQL user/password combination
, array(’user’ => ‘mysqluser3′, ‘password’ => ‘pass3′) // Third MySQL user/password combination
);
$mysql_user = $dbusers[rand(0, count($dbusers) - 1)];
$config['MasterServer']['username'] = $mysql_user['user'];
$config['MasterServer']['password'] = $mysql_user['password'];