You can turn that option on or off in the admin cp. vBulletin Options > User Registration Options > scroll to the bottom to set it how you like. You may also additionally turn this on or off for current users by going to Maintenance > Execute SQL Query and choosing one of the options from the Automatic Query dropdown.
Keep in mind for this last bit you will need to turn on the ability to run queries in your /includes/config.php file by adding your userid to the line in the section under:
****** USERS WITH QUERY RUNNING PERMISSIONS ******
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