Similar to what you asked before, I think what you want is to find the Vb element that is closest to half of Vb_ev, and then pluck out the corresponding pH element. The answer is very similar to what I gave you last time:
[~, idx] = min(abs(Vb - Vb_ev/2));
Vb_Mid_Pt1 = Vb(idx);
pH_Mid_Pt1 = pH(idx);
I think what's throwing you off is the distinction between values and indices. You are wanting to find certain indices in your array that have values near other specific ones. Hence the use of min with abs, which provides the index of the closest point.