"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.

Steve

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'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7