Assuming you’ve exhausted all possible ignition points of failure (including dropping in a new set of copper/nickel core spark plugs).

“ …Vacuum gauge readings are normal. Except that the needle vibrates so much I worry it will break the gauge…” by

That’s kinda interesting. I’m not sure how smooth the needle should be on a four banger but it starts to make me think intake manifold leak. Does the needle quiver at a frequency like once every other revolution or more like EVERY cylinder causes a quiver or is it impossible to tell? I could get real crazy and suggest a cam lobe is wearing flat. It would cost nothing to check.