Maybe the most correct way would be to define a vector for the original point and one for the difference to the other point. Let a, b, c, represent the x, y, z coordinates for the known point and d is the distance of point 2 below point 1, then something like
v1 = {a, b, c}
diff = {0, 0, d}
and then subtract the two
v2 = v1 - diff
However, to merely get the coordinates of course all you really need to do is to define
v2 = {a, b, c-d}