The host agent is not for Vcenter. It is for Monitoring Hosts directly or use them for OSSV.
For adding something to the vCenter you would need the OnCommand Host Package.
As I already mentioned, it is EOS, needs WS2008 and up to ESX 5.1
OnCommand® Unified Manager Host Package 1.3 for Windows® - Description
This page contains an overview of OnCommand Unified Manager Host Package 1.3 and its installation requirements for Windows. The Host Package supports the monitoring, backup, and recovery of VMware® virtual objects. This release does not include any GUI components.
When you are ready to proceed with the installation of OnCommand Host Package 1.3, click the link at the bottom of the page to go to the software download page.
Important: If you are installing both the Core Package and the Host Package, you must install the Core Package software before installing the Host Package software. After you install both packages, you must configure a host service with the DataFabric® Manager server before you can create your backup jobs. See the OnCommand Unified Manager Installation and Setup Guide for instructions.
The successor is the VSC
Unified Manager is not needed for the product.