Yours is an interesting dilemma. Have you tried disconnecting the booster vacuum hose, depleting the vacuum storage (15 to 20 hard pumps of the brake pedal) and then seeing if the braking is any better/worse? If there is no change with no vacuum, then you have isolated your problem. If there is a change, then you know that vacuum is not your problem and that it lies elsewhere.
Also was the vacuum checked at the brake booster for sure? Perhaps the vacuum line is collapsed internally? As well, even if you tested the vacuum at the booster you are checking the vacuum in a static condition NOT in the dynamic way the booster will work. The vacuum gauge has a very small volume so a pinhole passage in a collapsed hose will allow enough vacuum to trigger the gauge but will not allow enough volume to power the booster.
Then there is also the possibility of a bad vacuum check valve that was transferred to each successive replacement booster? The best way would be to install a T-fitting and the vacuum gauge AFTER the check valve. But with many heads involved and many new parts thrown at it, I would be surprised if it is this easy.