Vy equals 1.32*Vx

[Gysbreght]

The factor 1.32 applies if the thrust is constant and the drag polar is 'parabolic'. A parabolic drag polar can be written as:
CD = a + b*CL^2
where a and b are constants, CD=drag coefficient and CL=lift coefficient

Constant a defines the parasitic drag which is proportional to airspeed squared.
Constant b defines the lift-dependent (or 'induced') drag which is inversely proportional to airspeed squared.

Most airplanes exhibit a nearly parabolic drag polar at low speed and low altitudes where Mach effects are insignificant. At high subsonic Mach numbers the parasitic drag coefficient is no longer constant but increases rapidly with increasing Mach number, so the drag polar is no longer 'parabolic' and the 1.32 factor no longer applies.