Engine Vacuum vs RPM?

That's about right. A fixed load would be a smaller and smaller percentage of the engine's power as the rpms move up to the peak.

Vacuum is an excellent indicator of load relative to engine power. That's why it can be used as a reference for high load enrichment (step up or power valve), and adjusting timing for light load low density conditions (vacuum advance). Engines with computer based feedback fuel delivery use a MAP sensors fot the same purpose.