Vacuum is a special case of an insulator. Others will conduct beyond some threshold voltage; vacuum will not. On the other hand, vacuum will conduct at any voltage as soon as free charges are injected into it.

But electrons can still pass through perfect vacuums. Whether or not a vacuum conducts (obviously it doesn’t) is irrelevant.

I think that’s implied by his pointing out that capacitors use vacuum. A cap that doesn’t allow electron flow wouldn’t be a cap.

A capacitor shouldn't allow electron flow between its (dielectric separated) plates. The charges should build up on the plates, then exit through the terminals when discharging. If charges cross the dielectric, then you have leakage.

Hm. Apparently my Phys101-level understanding of electronics has caught up to me.

Caps don’t allow electrons through, do they? They resist DC, they only conduct AC?

That is correct. The dielectric ‘conducts’ an electric field, which attracts and repels electrons on the opposing plates.