I think there may be an issue with Filecache on the vB Datastore, make you should make sure the file has correct permissions: /includes/datastore/datastore_cache.php (chmod 0666 or 0777). As for vB Optimise, it's really a case of monitoring server load. Try leave it off for a while and watch your server load in the ACP for a couple of hours, then with vB Optimise on watch it, see if there are any differences (also make note of the sites current traffic when making these tests).
The filecache method may not be the best solution if your servers hard drive is slow or experiencing a lot of I/O reads (input/output). You should ask your host perhaps in they can install XCache or APC (preferably XCache), so you can use that method.