Dimmers that handle high current are not cheap. Typical consumer grade ones are 500W max, with most rated lower.
And dimmers made for dimming incandescent lights will deal with a purely resistive load, not an inductive load; the inductive load of a solenoid might not be enough to be a problem but...
Dimmers use silicon controlled rectifiers; the diode present on many vibe pumps may have an issue with that; iirc it just won't dim properly with a diode in line.
If you are going to hack your machine without doing any wiring, I'd question how effective you can really be.
Now, put an actuator on an opv, PID controlled by a pressure transducer, and NOW you're talking