VMware FT, App HA: What happens if vCenter Server is offline?

Some of this is covered in the vSphere HA Technical Deep-Dive...

But to summarize, vSphere installs HA agents on the hosts when you enable HA. vCenter is not required for HA to function. vCenter is needed in order to make changes to the cluster configuration, however, HA restarts follow a set prioritization schedule.

• In your VMware Fault-Tolerant (FT) example, the secondary would take over.
• vSphere App HA would restart the application or VM (assuming a supported application).

The point is that these things HAVE to be able to work without vCenter. E.g. the situation in which the host containing the vCenter fails needs to be covered (since it's highly-probable), and is.