<p>I want to fabricate a sample holder and shadow masks to use in vacuum chambers. The type of printing material is not important to me PLA/ABS/PC-ABS/nylon).</p>

<p>I'm worried that 3d printed objects (FDM) would degas under high vacuum. Is that an actual concern?</p>

Almost all of the FDM materials outgas even at normal atmospheric pressure, and, in fact, most plastics outgas.  Further, FDM and many other printing processes do not guarantee no internal voids - meaning that putting a 3D printed object into a vacuum may result in breakage, cracking, and possible explosion hazards.
For this reason I would focus only on SLA, as the model is printed within the liquid resin pool and should have a reduced possibility of internal voids.
Finding a resin that has a low out-gas rate after curing, though, is still going to be difficult.
For this to be answered more completely, you need to specify your tolerable outgassing rate, and the processes used inside the vacuum chamber.  For instance the answer would be completely different if you are discussing an electron microscope vs a sputtering chamber. As a start you might consider companies that specialize in
engineered materials intended for vacuum use
. They may be able to provide guidance as to which of their materials might be 3D printed and usable in your setup.