oil will pool in the airbox. and everything inside gets coated with oil. its just nasty and purely done for emissions from the factory. i doubt the airbox has much if any vacuum. vacuum should happen in the velocity stack and definitely after the throttle plate. if there is a large vacuum in the airbox your air filter is too dirty. i realize there will be a small amount, i prefer to vent mine to atmosphere.
lots of audi owners do this. they run the pcv into a catch can (oil separator) and then a line runs back to the factory location in the intake. a vacuum on the pcv will have a positive effect on power and provide greater differential pressure to keep the rings seated better reducing blowby. many big v8's run the breather into the collector of the header to use that as a vacuum source. moroso makes a kit to do it. i think the gains from airbox vacuum are too small and prefer to keep my setup clean.It seems to me the positive venting as designed would be more beneficial than just relying on natural venting. Now in a perfect world (in my mind) the breather hose(s) would be lengthened with an inline cooling point at the lowest point (think distiller), and include a tee / collector / drain; and still terminate in the air box to maintain a vacuum.