Mr Fox
When the certification of 'Concorde to be' was being discussed back in 1964 to 1965 time at Bedford (with the ARB in those days) it was decided to use the zero rate of climb speed because the slender delta did not stall in the conventional way but at VZRC you can only accelerate by lowering the nose and reducing drag thus giving away some height.
To the pilot such a procedure had obvious comparisons to the traditional stall.
JF