What are the reasons behind switching FDs off for a VOR approach?
ok two questios there really:

entering vapp as speed constraint at the FAF.
After the FAF you want to be flying a constant 3deg path (or whatever the approach gs is). Changing configurations and deploying flaps while on final will make the aeroplane baloon and deviate from that flightpath. To minimize those (uncorectable) deviations, the SOPs will call for the aircraft to be fully configured (flaps full, gear down and Vapp) at the FAF when starting the final descent.
By entering the vapp in fmgc at the faf you are basically telling the computers I want to slow down early and configure to full flaps before thr FAF. The fmgc will then compute your descent to allow for that deceleration segment. It's not SOP as far as I can tell, just a gimmick people use to tell computer you want early stabilized.

Switching FDs off
Two points here: you are not switching them off at the FAF. You are changing them to a TRK/FPA mode (usually 1nm from FAF) to better control your glidepath angle durring final. Yes, you could use v/s to do that, and use trigonometry to compute the v/s required to maintain 3 deg for a given groundspeed, but why would you.  Changing winds will change your groundspeed, and thermals will mess up your flight path. Setting -3deg in the FCU is much easier.  I can then set -3.2deg if I get, say 20 feet high, and I should be back on profile in about 1nm.
Second point: FDs will be switched off (completely) after disconecting A/P and at least at reaching minimas. Reason is the ap guidance is not certified to guide you after that so it is manual/visual flying using PAPI if available.


Note: this is all about flying a manually-selected non-precision approach (eg in NAV/FPA or TRK/FPA) mode with AP on. Using a fully managed approach is a different story