Most vehicles have a vacuum reservoir after 1980 or so for the EGR valve
You can tap into that to get a steady vacuum regardless of engine RPMs/Load
Or you can add an inline Check valve to a vacuum line to a specific device, like the by pass valve, they cost $5-$8 new
Check valve holds vacuum on one side
Vacuum is not technically "used up" by any vacuum device, once vacuum is applied it just stays at that negative pressure until released........................or leaks, pulls in outside air
Vacuum hose to intake is open so vacuum goes up and down
With inline check valve, vacuum is held at its highest point on one side, device side, until its released or leaks air in
So with a good seal it will hold a long long time
Power brake boosters are the same, that white 90deg fitting on the booster is a check valve, holds highest vacuum in the booster even if engine stalls, but also after vehicle sits for a few weeks, when you press on the brake pedal you still feel the power assist of the vacuum in booster even before engine starts
Vacuum reservoirs are used when a system will be applying and releasing vacuum like vent systems, or EGR system
When you apply vacuum to a vent "motor" to divert air flow to Floor then to change it back to Panel you need to "release" that vacuum to floor vent, so its lost
Reservoir allows you to "store" vacuum, like in the power brake booster, so you can release some and not lose all vacuum