I'm using esxi5.1.x and found the following solution to work after a couple previously mentioned solutions failed my case:
Log in directly to each host via vSphere despite them being technically managed by VC server. You'll get the popup that says it's managed (even though it's not showing up in vSphere via VC anymore...) so just click through that nonsense.
On the Summary tab, there's a section called Host Management in the bottom right. If your hosts are still thinking they're associated with the VC server, it should say "This host is currently managed by vCenter Server a.b.c.d and you'll have the Disassociate host from vCenter Server... option.
Disassociate each host one by one and when you go back in through vSphere via the main VC login, you should be able to successfully re-add each host without issue.
Hope this helps @esxi1979 and anyone else dealing with inexplicable vmware BS