[Lynne]

The datastore directory should be 755, but the datastore_cache.php file should be 777.

From here - The vBulletin Datastore:

vB_Datastore_Filecache
This option saves the datastore data in the /includes/datastore/datastore_cache.php file. Reading from the filesystem is generally less load-intensive than querying the database.

To use this option, you'll need to make sure that the /includes/datastore/datastore_cache.php file is writable and readable by PHP. Usually this is chmod 777. Then, uncomment the following line in the config.php file.

// $config['Datastore']['class'] = 'vB_Datastore_Filecache';