Super Jinni - Best alternative to XCache in my mind is Memcache. In regards to your issue it may be a CGI/FastCGI issue, do you know if you're using DSO or SuPHP for the handler? Typically cPanel let's you choose when building apache/php from within WHM.
Memcache would resolve the issues because it's different from your typical opcode cachers, it runs as a dedicated service
sadiq - Make sure vB Optimise is online and you have selected a cache system to use, with no cache system selected there's nothing to test