Just received a Vigor 2850n to go with my FTTC service from Plus.Net.
It took an email to Draytek to find this out, so I wanted to make it more visble.
(Draytek publish instructions for the V2750 here:
http://www.draytek.co.uk/support/kb/kb_vigor_v2750_setup.html)
The key information you need is that the VDSL service needs a VLAN Tag of 101.
You'll find the VLAN setting on WAN\General Setup\WAN1: set VLAN Tag insertion (VDSL) to 'enable' and enter a Tag Value of 101.
Of course, you have to enter your <username>@plusdsl.net and password (in WAN\Internet Setup\Internet Access: WAN1 Details Page).
Doing both of these from factory-reset gets my VDSL connected.
As far as I can tell, these settings will work for VDSL from other providers (including BT).