Ethernet is NOT operating system dependant and should make no difference what system is networked to it. As long as the BT voyager is already set up, just plug the new laptop into it and it will be fine. When they say not Vista compatable, they mean the driver software thats all.
I used to have this sort of set up and I would recommend dumping the voyager as they in my experience are really unreliable. Buy a 4 port wirelss ADSL modem and set that up to the BT telephone line. You will then have 4 ports plus a wireless network all in one box without hassles.
If you really must keep the voyager then just plug the laptop into it and use the internet, it will work fine. In fact it should not need any help to access the internet, Vista will sort it out. Just plug the cable in and click internet explorer.
If you need to set up the voyager again, just plug the cable to your old laptop, do the business and unplug it again.