No reason not to run an Ammonia installation at a vacuum suction condition...Lots of systems run 18" Hg all day long.
The consequence of air infiltration is, the purger has to operate harder or longer...
The normal large scale system practise is to employ Evaporative Condensers and a free draining High Side arrangement. The Condensing Temperature is thereby established as a temp higher than Design Wet Bulb, and the Dry Bulb is for most purposes, not a determinant.
ASHRAE 15 and IIAR-2 both establish Design Pressures for Pressure vessels, and by implication, test pressure for field piping, the factory setting requirements for Safety Relief valves, the Field Setting for compressor disengagement (high pressure cutout) and so on. Be careful with 150 Psig DWP for Low Side Vessels, it corresponds with a pretty low standby temperature so if the duty cycle is such that the operation will not be continual, the Low Side DWP should probably be 200.