in my experience with old motors - pre computer - once you cross the 240-245 at .050 duration line, idle vacum is not enough for factory optioned equipment without a vacuum assist of some sort. Storage canister or electric pump.
cruising the vacuum may build up enough to help brakes etc, but sit idle for a minute and less than 10hg vacuum won't assist much.
I can't imagine a cam the size your talking idleing clean below 1000 rpm

 

well i dont want to hijack his thread. i have a thread on here about map issues and vaccum.