It is possible for the intake gasket to leak on the underneath side and still be sealed on top side.
Has it started using any
oil?
When the carb was off, was there any signs of
oil in the intake? If it is a single plane you should be able to see into the center cylinder intake ports. That is the best place to look for
oil.
It wouldn't hurt to run a torque wrench over the intake bolts and see it they are loose. If any tighten up, see if it changes the vacuum.
If you replace the intake gaskets make sure they are metal reinforced. Victor something is excellent gaskets. I forget the spelling, but it is something like Victor Reinze.