I also have a problem with a new instance of vCenter and adding an already configured VSM to it. That's my szenario:
I have to install a new vCenter instance. VSM connected to it without any problems: same key fingerprint, new UUID. While I tried to import an ESX host, I always got the message that the portsets already exists and the CreateDVS command has failed. And that's the problem: I don't want to create a new DVS, I have to use the existing one, because it is in productive use. And therefore my question: how to add a configured VEM/VSM to a new vCenter instance without interrupting productive environment?