Mica heaters can be supplied to work in a vacuum environment IF they are burned-in or pre-conditioned to drive off any volitiles prior to being put into the vacuum environment. In looking at your space and heating requirements- a mechanically clamped, bulky mica heater assembly may not be the optimal heater selection. There are polymer thick film (PTF) and chemical vapor deposition (CVD) technologies that exist that may be a better fit for this particular application. Still these technologies typically are only cost effective if there are significant volumes associated with your needs. Good luck!
It is not the answer that enlightens, but the question.