The vac reservoir also aids the N112 valve (secondary air system to warm up cat quicker on cold starts) with its opening and closing, but as this is an emissions thing I have bypassed my N112 aswell (mainly becuase I broke one of the connectors on it and can't connect into the system).
In theory then, without the N249, and if you care less for emissions, the Vac res isn't needed.
This is because without the N249 to open the DV whilst throttle is open, the DV will only need to open when the throttle is backed off. And at that point there is vacuum in the system from the manifold, so it shouldn't need the reservoir.
What I want to know is does the vac res. do anything without the 249 operating?