A vacuum pump pulls a vacuum on the crankcase. The reduction in air pressure reduces drag from the rotating and reciprocating parts inside the case. This also allows piston rings of lighter tension to be used while maintaining ring seal. The HP "gained" is from recovering the power lost overcoming aero drag inside the crankcase and ring drag (friction) on the cylinders, as well as the potential HP increase from improved ring seal.
ring seal,for the most part, and keeping pressure out of the engine is always a good thing,and yes some windage bennifits also.but too much can cause oil pump cavatition. if the pump is sucking & the oil pump is sucking .......you can cavatate the oil pump..a dry sump should be used with it&multipul pickups are nice too.
A dry sump is not needed with a vacuum system. It won't affect the oil pump operation. The reason is because the oil system is completely within the engine case. The vacuum pump creates a vacuum on the engine case so both the "inlet" and "outlet" of the oil pump system is open to the engine case and it's vacuum. It is all within it's own atmosphere.
You don't want to run too much vacuum as it will starve the rings and wrist pins of oiling since the reduced windage stops oil from splashing and whipping around. I run solenoid valves in my vacuum pump system to operate at above 70% throttle or turns on over 4500rpm. With solenoids open it operates like an open breather system, hit the ga and the valves shut so it operates like a vacuum system is designed. Allows me to run the vacuum pump system on the street without any issues with regard to premature engine wear. Well, pump vanes wear out faster, but it's not that bad that I've noticed so far. Pretty much a non-issue so far.
I run Pauls (The Inspector) old vacuum pump. I switched to running a cooling system so I built a new mount for it. I also switched to pulling vacuum from the alternator stand. The catch can is mounted high enough now that I can have it drain back down into the passenger side valve cover when the solenoids open. I no longer have to drain the catch can after every pass, and can drive around without removing the belt (If you don't have it "deactivate" you want to only run the belt during a pass, and remove it for all other driving).
I'd run one on my dragster if I could find the room for it. Maybe someday.