One thing about gallium/galinstan - it would actually make a descent high vacuum seal as it has lowest vapor pressure of all elements - so it doesn't evaporate. The problem is that it sticks to just about everything that isn't PE/PTFE. Galinstan thermometers use some proprietary coating to make glass repel it. I was once entertaining the idea of using gallium for an electrostatically or MHD boosted Sprengel pump, but…
Pure galium has such a low melting point that it liquefies and acts like mercury if you hold a few grams in your hand. Probably non-ideal for vacuum tubes which do run a lot hotter than most other components.
Just keep it liquid, that's the point. If it freezes, it bursts the glass, as the OOP says.