Pro Install only covers actual installation of the mod into vBulletin, not the setup of the mod itself, although for vBO I tend to do a quick scan of the PHP Info and find out what they have installed and configure it to use that, or at least Filecache.
Are you sure XCache is configured correctly? Caching enabled, var_size > 0 and anything else Google has to say about XCache not working?
What happens if you create 2 separate php files on your server:
PHP Code:
<?php xcache_set('test', 'Working!');
PHP Code:
<?php echo xcache_get('test');
then run them in the order I showed here?
Fillip