Sorry DAR
I should have explained more fully. Vzrc is measured at full thrust in level flight. As you slow down below Vmd you have to add thrust until at some very high alpha - 35 ish - you finish up at full throttle and unable to climb or turn. This is Vzrc. The recovery from this is similar to a stall in that you dump alpha (and so drag), sacrifice a small amount of height, accelerate and fly away.
The snag with a multi-engined slender delta is that Vzrc is clearly thrust dependant. This is unlike Vs where there is only a small increase of you loose a donk. So one needs to consider Vzrc measured with one engine failed in order to handle the typical certification requirements of being able to continue to climb following an engine failure.
What margin (like the 1.3) you need to add to a 3 engined Vzrc would probably be type dependant. I don't know what was eventually chosen for Concord cert as I left Bedford to go hovering in 1967.
Hope this helps.
JF