Thanks for clarifying the ESXi host version (799733). That is likely not related to the release notes I linked. Unfortunately VMware generally tells us what issue is fixed in the release notes, but they don't always specify when it started. In your case though your current plan to move to a fresh vCenter is the right one. Let us know how it goes.
BTW, that ESXi version 799733 is really old. That's the very first GA release of 5.1. This version presents other risks to stability but I don't recommend upgrading just to resolve your current issue. Since you manually checked each host via the vSphere Client I would venture to say the problem is in the vCenter DB as others have mentioned above. Moving to a new vCenter is the easiest way to get around it without engaging support or hacking the DB.
Not sure if you have ruled this out as well, but ensure you have checked the templates by converting them to VM and reviewing edit settings. You might also choose to grep the vmware.log of all VMs for the UUID of the old dvSwitch, but that's probably overkill at this point. Again your plan to move the hosts to a fresh VC should take care of this.
