gesturgis wrote:

We have a group of systems that are connected to a "virtual intranet", i.e. a vSwitch that is not connected to a physical adapter. 

I think that VMware will try to make sure to never "break" anything from the VMs point of view with vMotion. If you have several VMs on an internal switch and one of those VMs by magic (vMotion) just disappears then any internal traffic flow will break, which is likely why this is not allowed.