Not enough vacuum?

Probably wouldn't be different from the two sources. Both are below the throttle plates but, the OEM ran it off the intake plenum. I tend to go with that.

Well, 15" at the source should cause you to bump your head against the steering wheel when you apply the brakes hard.
Decelerating in gear and decelerating in Neutral would not make any difference if I had to guess. Actually, decelerating in gear usually causes manifold vacuum to go higher. But, your road test suggests something different going on.

"The booster was just rebuilt by...." Rebuilt and overhauled are some of the most abused words in the English language.