If you are below min drag speed, then reducing speed increases drag. You have a fixed maximum amount of thrust available, so there will be a maximum amount of drag you can have beyond which level flight will no longer be possible. This drag figure will correspond to an airspeed - reduce further, drag will go up and you will go down. That is the Vzrc.
More engines=higher max thrust available, so more drag (slower flight) is possible in level flight. so Vzrc4<Vzrc3<Vzrc2, put the gear down and the Vzrc speeds go up.
Not a concept unique to Concorde, but the higher min drag speeds of the slender delta wing and its lack of a conventional stall speed (increase AoA and lift just keeps increasing) put the focus onto Vzrc as a reference for low speed flight.
Hope this helps.....