Impact on VMware environment when vCenter is down
Ok, so what happens with above features when vCenter is unavailable? The most important thing is: all workloads (VMs) should be running fine.

Central Management
Of course, when your vCenter is unavailable, you do not have the central management of your VMware environment. However, still all your VMs are running and if you need to administer them you have to log in directly to ESXi hosts (separately). So if you have 100 hosts and you need to do sth with 10 VMs, you have to log in at least once or max 100 times in the worst case...because maybe you would have to find those VMs.