I think both words have their uses and it depends on your point of view. Clearly we can say that an atmosphere of pressure is roughly 1 bar. Anything and everything less is reduced pressure. But is half an atmosphere a vacuum (not to most vacuum engineers. I used to work in the industry and a vacuum only really starts at 5% of atmospheric pressure. Of course if you are working with Hydrogen at say 500 bar a reduced pressure may be 100bar and that is certainly not a vacuum So reduced pressure may not be a vacuum, but a vacuum is reduced pressure from 1 bar.