FYI, If you are wanting backups of just the VM's, not application aware backups, then you don't need snapcenter.
I have snapcenter 4.2 deployed along with the databroker for application aware backups. I deployed the databroker, uninstalled the old VSC plug-in from SnapCenter and then added the databroker without issues. When you added the databroker to snapcenter what host type did you select? You need to make sure you select the type of vSphere and then put in the IP address and credentials you defined. Are you able to see the snapcenter plug-in in vCenter?