Oscillator circuit - Opamp
The begining of VA cutoff would be at the conducting point of the zener pair, equal to (Vz + VD).  The gain of OP1 needs to be calculated, (-R5/R4).  So the VB needed to reach the cutoff is (Vz + ZD)/(-R5/R4).  Feel free to finish the math.   

(This assumes that VB is the node to the lower right of label B.  This is not 100% clear as there is also a Viz shown there.)