Can you be more specific on the erratic behavior you're having? It may have nothing to do with the vacuum signal.

You can get away without the vac amp, but on a turbocharged engine it helps maintain smooth shifting across a variable boost range. Without it, the transmission will tend to shift firmer than it would otherwise since the VCV is not progressively operated.

You likely have a vacuum leak somewhere, something that isn't plumbed correctly, or a leaking ALDA. If the ALDA doesn't hold vacuum (indicating leaking seals), you will have some strange behavior since the boost signal affects the final vacuum sent to the transmission modulator.

So that you don't think you're going crazy, the vacuum signal to the modulator will not always be the same as the vacuum signal from the VCV. The boost level will control the final output to the modulator. Think of the vac amp as a summing amplifier, it takes 2 signals and compares them and sends the result down the line to the transmission.

Your input vacuum level does seem a bit low. Typically you should be somewhere around 12" at idle and without any throttle. Read the FSM on the setup procedure for the VCV and make sure it is adjusted correctly.

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