Bill,  As soon as you vCenter HA is robust enough, the vCenter can be hosted anywhere. We prefer it to be in the third site, but it is up to you.
I am working through the RPQ process for a VxRail Stretched Cluster implementation, and one thing I am seeing in the RPQ Excel document states the following:  "VxRail stretched cluster only support external vCenter and it needs to be deployed on a separate host on the 3rd site.  VCSA version is recommended."  I understand the requirement for an external vCenter, but is it strictly required to have the vCenter located at the third, witness site?  A better design in my opinion is to have the vCenter configured with vCenter HA (VCHA), with the active node at the "preferred" data site and the "peer" node at the "secondary" data site, with the vCenter witness node at the third witness site.  This way vCenter is highly available and located at the two most resilient sites, compared with being in a non-HA configuration at the witness site.  Is this a supported design for the external vCenter?  Or must it be located at the witness site only?
Does VxRail Stretched Cluster require the vCenter to be located at the witness 
site?