When you specify vif, you are creating a virtual interface, not a way to acces the network card.
The setup i have, is the machine has 2 pci network cards. Both using pci_hide, and i create 2 vif as below:
vif = ['mac=aa:00:00:00:00:13,bridge=dmz','mac=aa:00:00:00:00:12,bridge=intranet']
This means the domU will have 4 eth connections. 2 vritual, 2 physical. It does the routing and is also the dmz server.
Other domUs, I have:
vif = ['mac=aa:00:00:00:00:15, bridge=intranet']
Allowing that machine to have access to the virtual network connection, intranet.
I hope it answers your question. Someone please correct me, if I'm wrong
W