Pics of a 1.9 vacuum pump.
Here's my understanding of the mechanism:
The vanes and hub are a close tolerance fit to the lid and form a seal at the top and bottom:

Oil directly from the pump (~100psi) flows up through the shaft, fills the space in the top of the hub and the hollows behind each vane and pushes the vanes out to sweep the walls of the chamber. The sweeping from small area to large draws in air from the check-valve in the lid:

The vanes push the air (and oil that pushes past the vanes/hub) into the opening in the side of the chamber:

(Note the recess surrounding the bore in the bottom of the chamber.)
The opening in the side of the chamber leads directly down by the bottom of the bore to lubricate the gear (light can be seen below the bore):

The recess in the bottom around the bore receives oil from the hollows behind the vanes, its diameter is large enough to receive oil that is pushing the vanes in each slot. It supplies oil to the helical groove in the shaft. (It is the dark area that can be seen at the bottom of the slot):

The shaft is grooved to deliver oil from the chamber recess to the bore walls:

So, the vanes hold oil pressure, forcing it down into the helical groove. Any oil that gets past the vanes gets pushed out the opening in the side of the chamber along with the air drawn into the pump.
Taking out the vanes will let the oil flow unrestricted into the chamber and out the opening in the side. That could drop oil pressure the same way a bad seal at the bottom of the pump shaft could. Plugging the oil feed at the top of the hub will starve the shaft and the drive gear at its bottom.