The clue to me is that you found the initial timing retarded by 4 degrees, so somebody was trying to get it running right without pinging by backing the timing down.
First I would make sure you have the distributor stabbed correctly. Set no. 1 at TDC compression and make sure the rotor is pointed toward the no. cylinder (at the cap).
If not, the camshaft timing may be OFF, which can also account for you low vacuum readings. You'll have to pull the timing cover to verify that.
Could have just been a poor rebuild job as well.
excellent point on the camshaft, I'm suspecting the "rebuild", but I wouldn't panic, could be somewhere they made a mistake like the camshaft not properly lined up with the crank, intake gasket not sealing, etc etc etc
all of my Pontiacs have had excellent vac so soemthing is amiss
