Your first statement about diamagnetism is correct, orbiting electrons create a magnetic field so as to oppose the external field.
If an atom contains uncoupled electrons, their magnetic moments will align either parallel or anti-parallel to the external field. Because laying parallel (in the direction) of the field is the lowest energy configuration, the majority will do this, thus their magnetisation adds to the external field. This is known as paramagnetism.
Paramagnetism and diamagnetism only exist in the presence of external field. The relative sizes of these two effects depends on the substances involved so I couldn't comment on how comparable they are to a vacuum. How close to a vacuum do you need them to be? At what point would you no longer consider it significant? It depends on what you're doing!
There is also nuclear paramagnetism, but this can be thousands of times small than other effects so difficult to measure.