What is the formula for heat of vaporization under reduced pressure?
You are looking for the Clausius-Clapeyron equation.  It comes in various forms:

The exact version is

$$\frac{\mathrm dp}{\mathrm dT}=\frac{\Delta H}{\Delta V T}$$

or, since the volume of the gas phase is so much larger than the volume of the condensed phase

$$\frac{\mathrm dp}{\mathrm dT}=\frac{p \Delta H}{R T^2}$$

or even more succinctly, assuming $\Delta H$ remains constant

$$\frac{\mathrm d \ln p}{\mathrm d \frac{1}{T}}=-\frac{\Delta H}{R}$$