The vacuum at the hose is normal and is from the engine's aspiration.
Opening the cap relieves crankcase pressure same as a functioning PCV should.
I believe there is a baffle in the rocker cover to prevent oil spray from entering the PCV system, perhaps this is clogged pressurizing the crankcase and blowing oil past the seal. Check for pressure at the port on the cover for the hose with the cap closed, it may only be slight. Compare with the opening for the filler cap. If you have more pressure builing at the filler cap I'd look into a restricted baffle.