Now I am confused. Earlier I thought that VPN-ID was meant to be like a traffic selector, i.e that you could create one IKE SA and then for each IPsec SA you select which VPN-ID is used. You seem to be proposing that VPN-ID is more like the IKE authentication ID, i.e the identity of the other end.
For that kind of use you need to have separate IKE SA for each VPN, and then the proper way to do that is use separate credentials and authentication ID per VPN.
Anyways, I think this is something that is not for general IPsec use, but more specific case, thus I do not think we should include the current issue #68 in the RFC2401bis now. I think we can write new document to describe how to do that kind of things.
Can we agree on that now?