In a physical vapor deposition process for fabrication of perovskite absorber layers it is an important prerequisite for large scale application to understand the reaction steps and the impact of the deposition parameters on the structural and compositional characteristics of vacuum deposited lead halide perovskite layers. In the sequential deposition method, we found a narrow and low lying temperature window for the transformation of a vapor deposited PbCl2 base layer to CH3NH3PbI3 perovskite by reaction with evaporated CH3NH3I. To modify the deposition parameter window, we systematically varied background pressure and substrate temperature and analyzed the resulting composition using X-ray diffraction, energy-dispersive X-ray spectroscopy, and scanning electron microscopy. The resulting reaction products and byproducts CH3NH3PbI3, CH3NH3PbCl3, PbICl, PbCl2, and PbI2 were identified and the variation of their concentration in dependence of background pressure and substrate temperature is explained by discussion of a set of reaction equations and reaction mechanisms on a thermodynamic basis.