Not a specialist but you don’t want to take a deep breath before going into hard vacuum. The air in your lungs will try to get out and break membranes and blood vessels which will kill you.
Our bodies rely on external pressure to not deform itself and kill us in the process, we’re really « well build » for our environment
15 seconds in hard vacuum seems plausible to me but for Mothership TTRPG the rule should be « whatever make a good story »