@softvnzla & smnoel
Please run the System Test within the Admin CP (under vB Optimise) and let me know which tests succeed or fail. Generally if flush cache is failing and you're running XCache you'll need to enable XCache configuration within vBO options and also edit ./vboptimise/config.php to specify your XCache username/password (or alternatively disable XCache admin authentification within php.ini).
If it's eAccelerator, you'll need to provide the script access to run the flush command, more information:
http://eaccelerator.net/wiki/Setting...0.9.5andhigher