I can only speculate, but it sounds like the vApp settings may have been lost due to a change in vCenter Servers, the vApp being broken apart or the VMs moved out of the vApp since the initial deployment. I would open a SR with support if you can.
In the case of a broken vApp, check the vApp settings in the vCenter Web Client. If they are gone, the DB may be unable to start. If this is the case, try the following:
On the vPostgres VM edit the following file:
/opt/vmware/etc/isv/subsequentboot
Replace the following string with the username under which the DB processes should be running.
$(ovfenv --key DB_USER)