The vacuum plays an important role as it insulates the tube from the outdoor temperatures.  Thermomax has the highest vacuum level of any tube.  When removing the matter from the tube, the vacuum is taken to 10-6 milibars.   What this means in real life is almost all mater was removed providing excellent insulation for many years.  In addition the Barium getter that traps any remaining matter is large, thus more effective, than in many other tubes.  Thermomax uses soda/lime glass that prevents the transmission of outside gasses through the glass that would degrade the vacuum over time.