Each vSAN stretched-cluster configuration requires a Witness host. The Witness must reside on a third site that has independent paths to each data site. While the Witness host must be part of the same vCenter as the hosts in the data sites, it must not be on the same cluster as the data site hosts. The Witness ESXi OVA is deployed using a virtual standard switch (vSS).
A vSAN Witness Appliance, or a physical host, can be used for the Witness function. The vSAN Witness Appliance includes licensing, while a physical host must be licensed accordingly.