MK940
Yes, correctly worked out, Vzrc is the Zero Rate of Climb speed.
It is the speed at which the total available thrust exactly balances the total drag, and two assumptions are made. Firstly, that the (remaining) engines are all producing full power, and secondly, that the aircraft is clean with no unexpected drag.
So, when quoting a Vzrc speed, we can qualify it further by stating the number of engines that are operating, as Vzrc will increase with a reduction in the number of operating engines.
This is what NW1 meant by the terms Vzrc4 (all engines operating), Vzrc3 (three engines operating) and so on.
We can further qualify Vzrc by talking about extra drag, and for Concorde principally this is about whether the landing gear is up or down. We end up with terms such as Vzrc2 Gear Down.
This is the minimum speed at which Concorde could maintain level flight, on only two engines, with the gear still down, and is calculated for every Concorde departure.
If the gear could be raised, then total drag would reduce, there would then be an excess of thrust over drag, and the aircraft would begin to climb.
Regards
Bellerophon