Microsoft VSS libraries are a smart way of doing backup, in order to guarantee a consistent backup of applications supporting them.
Veeam can do backups without the help of permanent agents inside the virtual machines, anyway if you want to do a VSS-based backup, Veeam deployes (and removes when the backup is completed) a small agent in the Windows VM in order to cohordinate VSS tasks. Usually, this is done by connecting to the target VM via network.
But, what happens when for any reason Veeam is in another network of the one used by the virtual machine, or they cannot communicate to each other?
In these situations Veeam can leverage VMware VIX libraries, that allow for direct interaction between the guest operating system inside a VM using the hypervisor running it, without any need for network connectivity.