I can't say I have the answer, but the pump's relief valve balances discharge pressure against ambient (external) pressure.
If the pump is external and sees atmospheric pressure (14.696 @ sea level), it won't change with reduced crankcase vacuum.
If the pump is inside the crankcase, the relief valve sees vacuum, which reduces the pressure against the relief valve, lb. for lb.
If the pump is big enough, viscosity high enough, blah, it won't affect pressure at the bearing at racing RPM: the expected pressure minus the vacuum is still a safe figure.

Did I miss something?