When VMs are placed on a private virtual switch they have no external physical NIC so are generally not allowed to be moved from one host to another with vMotion. You must first power them down then power them on.
This impediment implies that private test labs and those virtual switches that sit behind virtual firewalls suffer from this in ability.
Yet there is a solution. If you modify the file C:\Document and Settings\All Users\Application Data\VMware\VMware VirtualCenter\vpxd.cfg with the following additions before the closing </config> you can enable vMotion for these virtual networks.
Now all your VMs will participate within vMotion, but just remember that a private network is private to an VMware ESX host and NOT an VMware Cluster. Therefore if you migrate one, you will have to migrate them all if they need to intercommunicate.