have you checked the check valve on the vac pump. Just unscrew the lid and test/clean out the check valve. I saw a thread somewhere where they replaced that check valve with on from mcmaster.
BTW, that check valve on the top of the vac pump just presses in and can leak around the base. I put some silicone gasket maker around the base of mine because it was loose, think it got hit or something from the PO.
Low oil pressure when hot is normal, you'd have to give some numbers.