You need to create a scheduled task that runs once an hour then use some PHP along the lines of:
PHP Code:
// You need to create a setting field that holds the current value
// You don't really need this line I just added to make it easier to see what is going on below
$curval =& $vbulletin->options['settingcurval'];
// Explode the comma seperated values into array
$settinglist = explode(',', $vbulletin->options['varsettingname']);
$curkey = array_search($curval, $settinglist);
// Move the key along one
$curkey++;
// Set the new value
$curval = $settinglist[$curkey];
// RUN A QUERY TO UPDATE SETTINGS TABLE
// To update the $curval into the database.
// Includes adminfunctions.php
require_once('/includes/adminfunctions.php');
// Update the datastore
build_options();
Something like that I think should work, there might be a better way to do it. But that is the way I would have done it anyway