There have been multiple companies coming back to try to make small batches of vacuum tubes.

It’s not a mysterious process that depends on arcane knowledge. It does require some tooling and process refinement, but the only real obstacle is getting enough demand to pay off the investment in tooling and process refinement.

It's basically the same process as making an incandescent lightbulb. Which is why vacuum tubes took off in the first place. It's more parts and more metal working, for sure. But really it's shaping and dealing with thin films and a vacuum chamber.

The hard part is precision.