How can I connect to vCenter's PostgreSQL database if the 'postgres' user has a default empty password?

If the 'postgres' user password is not set, you can connect using `psql -h localhost -U postgres`. However, if a password is set and unknown, you can use the 'vc' user by reading its plaintext password from `/etc/vmware-vpx/vcdb.properties` and then connecting with `psql -h localhost -d VCDB -U vc`. This method is detailed in vSphere Development Guide 4 - PostgreSQL.