OOTB there is not a great way to do it. The best way would be to install the EPO agent onto your vcenter/s Doing so will allow you to monitor a service (VPXD) once you set that up there will be a resource availability metric that you can add to your report that will give you the up time of the service.
This is better than monitoring the VM as the vcenter service could be offline but the VM is still powered on. This also opens up the ability to alert on service failure.
couple of thing to be aware of
In the below the top graph is a windows vcenter service monitor i created and the web client is an OOTB eops agent service monitor. You can add this to a list view and set an average over what ever time frame you need