Great news, I was beginning to worry.
While a cache prefix is created by config.php, VaultWiki currently has no specific implementation for Memcache. It is one of those features we started and never got around to finishing. Currently VaultWiki will just use whatever cache your forum uses, which for XenForo is typically the xf_data_registry table.
Now vBulletin does offer an option to use Memcache for vBulletin things, and I believe if that is turned on then it will just offload VaultWiki caches there. I'm not sure if XenForo has something similar or if there is an add-on. There is a known bug we discovered early on in vBulletin's Memcache implementation where it is impossible to remove entire keys from the cache without circumventing the forum. This did cause problems when a reinstall was performed or when VaultWiki was upgraded from a Lite version. However, VaultWiki's own cache prefix is never used for this case, because the main Memcache instance is controlled by vBulletin which has its own cache key.
Changing this to a bug and marking this as fixed in the next release. When uninstalling on vBulletin, VaultWiki will now workaround the vBulletin bug and delete the Memcache entries directly. Again I don't know that XenForo offers Memcache support at this time, so this fix only applies to vBulletin.