The VDEV is with respect to the FMGS Computed Vertical Profile (DSC-22_20-50-10-28 P 131/162). This is the managed speed shown in the PERF DES page.
On the Thales systems (and newer standard Honeywell systems), you will be able to modify this speed on the PERF DES page directly. In which case you don't have to resort to some of the old tricks that existed (i.e. re-enter CRZ FL, modify Vert SPD LIM etc).
Remember, the VDEV is not going to be very useful unless you are getting down to your final altitude at high speed & decelerating from that point. If you are configuring your aircraft earlier due to an ATC speed limitation and trying to keep a continuous descent going, the VDEV is going to show you deviation with respect to levelling off and decelerating.
Last edited by radical_100; 12th Nov 2020 at 08:35.
Reason: References, Clarity