Odd, I posted this once, and it didn't show up an hour latter, some sort of server thing maybe???
anyway:
First off, the wording used in that chapter doesn't say vacuum suffocates, it says your veins burst etc. Description wise, simply being able to breath isn't going to stop you from making these CON checks. Unless you got seal space suit, your body is being ripped apart. The mechanic is based around suffocation, because that is a very nasty system. It doesn't inflict damage (which would by all rights allow a high level character to walk through vacuum like it was a Fire-wall: painful but survivable), and it doesn't use saving throws, which could make high level characters nearly immune to it.
Part of the trouble with vacuum doing cold damage is that vacuum would then damage EVERYTHING, including constructs and undead, which by all rights I would prefer to have immune to vacuum damage.
Look at it this way: Vacuum seems cold because it your body's atomic bits and parts have nothing to bounce against and temperature is technically a measurement of kenetic energy... I think that temperature you have is around 0 Kelvin (that's absolute zero) but space can also be extremely hot, because there is nothing to stop the sun's direct rays and radiation, so anything without cover is going to burn up as they suffer vacuum damage.
I'd prefer to avoid and overly complex system of vacuum damage that blinds, freezes, and possibly burns, because it becomes to much of a pain to designate what is immune to vacuum damage (a Stone Gollem, Iron Gollem, skeleton?) and what isn't immune. Also, I want to make sure vacuum damage isn't something that can be avoided by using standard energy reduction and immunities.
Lastly, I want a system that isn't necessarily going to wipe out a whole group of low level players instantly should a single encounter go bad.
I will admit, that at first Subdual damage doesn't seem right, but look at the damage space is doing under our rules: CON checks, increasing with each check. This isn't a Fort save, so special saving throw features of class levels don't help. A more level playing field. As soon as you fail, you go unconscious, and 3 rounds latter you die. PERIOD. Should you some how make it through all that anyway, you've got to worry about the subdual damage. That damage can render a character unconscious rather quickly (skipping to that part of the vacuum exposer) and will at least keep a character that survives vacuum feeling its effects for a while.
This method keeps vacuum from being side stepped through cold protection spells scuba gear, but it also doesn't make it instantly fatal (although that is probably more realistic, but this is a fantasy game after all)
