I would expect more, but that could be an issue of installed centerline or preload or ignition timing, pcv, etc.
Idling too rich or too lean can cause a misfire too which may result in lowered vacuum. A lean misfire can read rich on an O2 sensor and a rich misfire may cause a lean read.. Sometimes it's counterintuitive.
A compression test could rule out a leaky valve or cam that's going away or a bad lifter. The plugs seem to suggest #3 might have some issues. Headgasket could be at play, or even the slightest exhaust leak could cause a poor afr read and cause issues. Hard to say at the moment.
