If we're talking homemade vacuum tubes... I wonder if it wouldn't be easier to just use metal endplates with feedthroughs for electrical (like spark-plugs) and with v-grooves for o-rings or some other gasket material. this kind of construction can handle vacuum easily, I think?

It will not hold a vacuum. Gas can diffuse through rubber and rubber itself will out-gas in very low vacuums. Metal and glass seals are the only truly air tight sealing methods. Even then, remaining gas is trapped in tubes and a getter https://en.wikipedia.org/wiki/Getter is activated at factory to trap this remaining gas on the tube wall in a thin layer of metal. That is why many tubes have a dark silver coating at the top or a band, that's the evaporated getter metal.

See also: https://en.wikipedia.org/wiki/Materials_for_use_in_vacuum

Also, don't use V grooves for O-rings. Read the Parker o-ring handbook for proper seal design: https://test.parker.com/content/dam/Parker-com/Literature/O-...