At what altitude do vacuum powered gyros become ineffective?
Just like altimeters, it depends on the specifications.

The manual for this random vacuum attitude indicator says up to 40,000 feet.

(Image source)


  In an aircraft, the vacuum source is often used to power gyroscopes in the various flight instruments. To prevent the complete loss of instrumentation in the event of an electrical failure, the instrument panel is deliberately designed with certain instruments powered by electricity and other instruments powered by the vacuum source.— Wikipedia


In a piston engine airplane the vacuum pump is geared to the engine.

Historically electric motors weren't reliable, that has changed now of course.

At 80,000 feet the air density is only 14% compared to 40,000 feet. I highly doubt it would be feasible (size, weight, RPM) to create such a pump. And I'm pretty sure it's of no use to jets.