A very reasonable and clear question .... A vacuum tube of course does not have a vacuum in it ....
In an Absolute vacuum the only way for current to move is by the electrons emitted by the -ve terminal ...
the hotter the temperature of the -ve terminal , the more easily electrons "boil off" and they will travel to the +ve terminal and cause a current .. but I would imagine this flow would be extremely low ... if there were molecules in the space they would ionise at some point and cause a large current flow , but there are none .
In an Absolute vacuum the only way for current to move is by the electrons emitted by the -ve terminal ...
the hotter the temperature of the -ve terminal , the more easily electrons "boil off" and they will travel to the +ve terminal and cause a current .. but I would imagine this flow would be extremely low ... if there were molecules in the space they would ionise at some point and cause a large current flow , but there are none .