Not volume as in flow. More like storage volume.
I think this is correct. They are similar to hydraulic accumulators.
The reservoirs even out the pressure in the system when an event occurs like the solenoids open fully to spool the turbos or when the brake pedal is pressed. Without the reservoirs the vacuum system volume is small so any flow to the waste gates or brake booster would result in a larger pressure change in the system until the vacuum pump can catch up versus a system with reservoirs. I don't think it is a mistake that they are placed close to the solenoids.
They are like capacitors in electronics. Capacitors store electric charge and resist changes in voltage. One way they are used is to keep voltage consistent when changes in current, or flow of electric charge, occur. The vacuum reservoirs keep vacuum system pressure consistent when changes in vacuum flow occur.
I agree that if the reservoirs are replaced, the replacement should have the same volume. I don't think it is important if that volume in made up by one or multiple reservoirs.