MV comes in right away and is reduced or eliminated as needed under load. The heat and combustion, The retarded timing at idle in a pv engine creates heat that burns the fuel more efficiently. It's does run cleaner just sitting in traffic if that is your goal and some engines with big cams and vacuum issues cannot run MV.
Performance, Some cammed engines need the advance to idle well. One of my engines runs like crap without the vacuum hooked up MV and is soggy from a dead start down the road not track. Full performance should not be affected as the vacuum goes away under hard throttle.
I'm sure I missed stuff and I'm sure to get corrected too. Go. ;)