Probably something closer to MPLS all things considered. If you think about it they effectively are a carrier network with many virtual customer networks on top.

MPLS has a bunch of advantages over VXLAN and is decidedly L3, which is more suited to AWS VPC. The real question however is what the control plane looks like.

They may just go pure carrier style and just do tagging and resource control on the NIC but it's entirely possible for the NIC to be doing much more, especially if they have a dynamic control plane similar to OpenFlow (though probably not exactly OpenFlow.. it's less useful than it sounds).

I would love to see a talk on AWS VPC internals rather than sitting around taking pot shots. :)