Did you ever get this working? I am having a similar issue. If I deploy my 2012 r2 template using custom specs it never finishes because after deployment the NIC becomes disconnected despite me having 'connect at power on' checked. If I deploy WITHOUT custom specs the NIC connects fine. I am using VMXNET3.
My old templates (2008r2 and 2012) both work fine when deploying from template using custom specs. I have another post that I plan on updating as I figure things out...
deploy from template with custom specs not working - NIC becomes disconnected