The virtual appliance you can download already has the correct udev rules which usually cause this situation (see here). There is a file in vyatta which has a record of the specific interface information. When you migrate the machine, this record causes the machine to think the adapters have changed. Edit the following file:
nano /opt/vyatta/etc/config/config.bootYou can either remove the interfaces section or edit the interfaces to suit i.e. change the MAC addresses etc.