I recently helped answer a question internally about the default credentials to the VCSA 5.1 (vCenter Server Appliance) vCenter Server and SSO (Single-Sign On) Database for troubleshooting purposes. I thought I share the details in case this might help others.
With the release of the vSphere 5.1, the VCSA now runs VMware's vPostgres database for both the VCDB and SSODB. You should also know there is no default credentials for the database as the passwords are automatically generated during the initial application install and the database password will be unique on every VCSA.
However, you can still retrieve the password for both the VCDB as well as SSODB (this took a bit of digging in the appliance).
Disclaimer: You should not have any reason to go into the actual DB of either vCenter Server or SSO other than potential troubleshooting with VMware Support. Please use caution if you do choose to connect to the DB, as you can potentially impact your system.
You can view the credentials for the VCDB in the following file: /etc/vmware-vpx/vcdb.properties
Here is a screenshot of the file content as well as using psql client located on the VCSA (/opt/vmware/vpostgres/1.0/bin/psql) to connect to the VCDB:
You can view the credentials for the SSODB in the following file: /usr/lib/vmware-sso/webapps/lookupservice/WEB-INF/classes/config.properties
Here is a screenshot of the file content as well as using psql client located on the VCSA to connect to the SSODB: