Hi,
I recently noted strange behaviour. My vCenter work perfectly but something strange happens when connection to internet and DNS server losts. I just shut them down temporarily. vCenter processes/services stopped (I have 6.7 appliance and seen that from vCenter management web, in 5480 port) and web client wasnt accessible. After I restored DNS server and internet, vCenter after boot works again normally. I dont use names, all ESXi-s and vCenter by IP-s only.