At the moment there is one phone line coming in with ADSL on it. I assume that when fiber is connected, they come to the office, put a new fiber cable into the building from the street, add a new socket for connection to the new router and the phone line remains for POTS.

in most case VDSL is fibered to the distribution box then converted to the standard copper single pair line for both phone and VDSL

in my case ( Manx telecom - isle of man ) the VDSL is fibered at the exchange and copper from there if the client is further than 2km from exchange then they can't have VDSL which is my case as well so stuck with ADSL2+

I may have missed the point of the Set up but why not try a 2850, the rj11 on that is VDSL and then use the WAN2 port with the 120 for the ADSL and have dual wan (fail over or load balance)