First of all, lets agree on a fact: there is no friction loss in a vacuum; this is so because there is no matter existing in a vacuum
. So the statement of losses for air in a vacuum is illogical. What you must be referring to is a partial
vacuum. Now, that is a situation where you have partial matter involved and that is what is required for identification in order to arrive at a pressure drop.
If you have the required basic data identified: fluid, flowrate, density, temperature, viscosity, diameter (of duct, pipe), etc., etc., then you should be able to simply apply the conventional fluid flow equations in combination with the expected friction factor. However, be aware that the usual inaccuracies and errors in the relationships are now applied over a very narrow range: approximately 14 psi. Therefore, if you are looking for calculations that will reveal friction losses in the range of fractions of a psi, I wouldn't trust them. I can't imagine what you application is that would require such precise calculations and you haven't told us. Therefore, I can only assume that you need to estimate the friction losses and these will always be well within the range of no greater than approximately a psi. If that is the case, why would you require to have the calculation?
Could you please be more explicit with what your application is and what your basic data looks like? Air in Denver Colorado, Mexico City, La Paz Bolivia and such sites is at partial vacuum (a partial vacuum defined as pressure less than an atmosphere, 14.696 psia); a vacuum (0.000 psia) doesn't exist in real, empirical industrial applications. So, if you are just circulating air through a pipe in Denver, you have partial vacuum conditions exposed to a friction drop for the air. This could be handled with the Darcy-Weisbach equation.
There are published air tables for such applications; I'm not at home at present so I can't cite the sources. But I'm sure other Forum members will mention them.