Is it for functional or safety distances?
Normally the parts inside the vacuum tube would be functional, so you can base the distance on widely available research on breakdown in a vacuum (e.g. one paper I found gave a figure of ~45kV/mm, wikipedia gives 20-40kV/mm depending on shape/material, both of which far exceed the value for air which is around 3kV/mm).
Since there is no dust, pollution etc in vacuum, the distance would purely a function of clearance, not creepage.
Although some parts of the tube might be earthed, the circuit should be such that under failure conditions, the currents will only circulate inside the equipment, and cannot get outside.