"Should the vacuum port on the fuel pressure regulator hold vacuum if testing with a MightyVac?"
I don't know - but it is more a 'vent' port than a vacuum connection. It connects to the breather tube, and as such is not a component in the manifold vacuum array. I.e., it will not cause any mixture metering error. My guess is that your test produced normal results.
'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7