To add to Dudley's comment, one might have a suitable pump (rotary, diaphragm, scroll, etc.) running all the while with a variable constriction on its inlet, allowing one to adjust the reduced pressure that the pump presents to the system.
Dudley J Benton, so the steam ejector or the mechanical vapour compression that is usually reported in the literature is a heat integration process and not a necessity to maintain vacuum in the effects?