I never used a vacuum holder (and never made shots of the night sky larger than 6x6). But it is quite clear that the vacuum pump has to work during the exposition. Such exposition may last an hour or longer. WIth such long exposition times a lot of problems occur. One of them is the movement of the film due to its exposition to falling temperature and moisture.
I recommend starting with 6x6 or even with 35 mm for looking what happens and what fails. I further recommend to use a high speed film such as Ilford 3200 (which is an 800 ASA film in fact). The you may get decent results after 10 ... 30 mins of exposition, depending on the sky quality of your site. You fill find a lot of problems even then. Some are problems with guiding and optical aberration of a rather wide open lens.
If you start with all problems at once (large format = small stop = extra long exposition time, medium speed film = even longer exposition time, heavy equipment = more problems with the mount) it may be that you give up too early.