The Agent for VMware Virtual Infrastructure is licensed per physical host, with unlimited guests. There is no "agent," per say, Symantec is leveraging the recommended method from Vmware in using VCB (native technology from VMware) to backup the virtual machines. Once the license key is entered, the 12.5 install can now talk VCB, which in turn will talk to the VC or ESX host.
The Sharepoint agent is licensed by sharepoint server. If you have a separate SQL installation from the Sharepoint farm, you will need an Agent for Windows to backup the Windows installation for the SQL box. The SQL license is part of the sharepoint license (at least they give you that...)
If you are virtualizing Sharepoint, the agent for VMware will cover your OS installation. But you will lose the benefits of the sharepoint agent and the GRT functionality it provides. But then again, just restoring the entire VM, can get you the individual files, just like GRT I guess?