Hi
One of the pre-requisites for VOVA is a single datacenter in your vCenter Inventory. This is a temporary limitation due to a bug upstream https://bugs.launchpad.net/nova/+bug/1180044. As soon as the bug is fixed, we will provide the patch that can be pulled in to your environment.
Regarding the OpenStack Dashboard overview, it displays the default tenant quota that is set by the administrator. To view the actual resource reported from the cluster, please check the Admin-->Hypervisors page. Note:- The reported memory will be slightly lesser due to the hypervisor overhead.
P.S :- I will update shortly with the CLI to change the default tenant quota.