BT Infinity is installed now and working fine with their equipment supplied. However, I need to get the 2850N connected again to use the VPN feature.

I tried taking the DSL cable from the OpenReach Infinity box and putting it into the VDSL port on the 2850 but although it seemed to connect in the router status, (gave an upload and download reading in green) it did not show connected on the computer and i was unable to browse any websites.
I have uploaded the latest firmware 3.6.4

Has anyone else had this problem.

Should I just leave the OpenReach box connected and take the ethernet cable which is going to the BTHub and put that into the WAN port (4) on the 2850?

have you enabled the VDSL mode on the 2850 you should be able to just plug into the RJ11 port without the BT Hub

Yes, I have enabled VDSL.

as long as you have the right tag etc then it should just work
the one above have worked fine so over a year now

I will double check the tag and try again.
What firmware are you running?

this is the latest if will help you but use the *rst* file to set it at factory defaults which may helps if are bugs
ftp://ftp.draytek.com/Vigor2850/Firmware/v3.6.6/