We have a remore worked who has a Vigor 2800 and wants to get BT Infinity installed (as his normal ADSL connection is not very reilable).

We use LAN to Lan VPN so its crucial he has a Vigor router (and not the stock BT router)

Are the 2800 and 2820 comptaible with fibre broadband?

Is there actually much difference in the way the are configured to connect to the internet, if any?

The Vigor 2800 is not compatible with Fibre as it only has a single ADSL2 WAN port. The Vigor 2820 has a Ethernet WAN port that can be used to connect to a Fibre modem. Unfortunately he will need up upgrade.