The plane of quasielemental vacuum is a thinning of the atmosphere of the plane of Elemental Air until a true vacuum is reached. Paradoxically, pressure and temperature do not drop as the atmosphere thins. Those who venture here must contend not only with nothing to breathe, but worse yet, nothing to transform into a breathable atmosphere. The ambient light of the plane of Air becomes dimmer and dimmer until a wall of solid blackness rises before the traveler - the Negative Energy plane.
An unprotected character in the plane will die of asphyxiation. This plane is the prelude to ultimate death, a glimpse at the fate of the failed petitioner, because this is the plane of Nothing. Leaving the banks of Air, the atmosphere here becomes less and less until there's nothing - no breath, no light, no sound, no warmth.
For certain mystics - the ascetics of the Doomguard and the dancers of the Dustmen - the plane of Vacuum is the ultimate goal. It's the last door of the mystery, the kiss awaited by all. Such sages spend their lives mapping the doors of Sigil until they find the path to Vacuum and then they pass through, never noting for others what they've found. It is Vacuum, it is right.