Hmm.....
You can try running this command.
mccli client add --type=vcenter --virtual-center-username=$VCENTER_USER --virtual-center-password=$VCENTER_PASSWORD --name=$VCENTER_ADDRESS
Then restart all the services...
su - admin
mcserver.sh --restart
exit
emwebapp.sh --restart
If that doesn't work you might be better off deploying a new VDP appliance. Last time I had a problem like this I was able to get VDP back up and running, but it took a lot of effort and still acted funny. I ended up rolling it's checkpoint back and that resolved my issues completely.