Scott,
Forget double NAt, I just meant that you can do great things to get around duplicate IP addresses etc.
I am not sure where your difficulty lies. If you have created the VLINK then on each vdom that the vlink is between, you have a new interface.
Routes and policies can be applied to these as if they were any other interface.
What is confusing you is probably that the gui has no support for VLINK interfaces so you have to set them up in the CLI. STarngely enough once you have created them via CLI you can modify and edit them via the gui.
This lack of support goes through to Policies and Routing as well, so create a minimal config for each of these in the CLI eg allow all from internal to vlink0. This will then show in the gui and you can do more with it.
Jon
Still learning to type " the"