As the Millennium Falcon is being sucked into Death Star Bay 327 in A New Hope,
we see a pair of stormtroopers, standing in hard vacuum, gawking at it. In contrast, the two Death Squad Commanders at a console seen just seconds earlier are clearly behind a window. The ease with which the stormtrooper helmets otherwise appear easily removed and open around the collar notwithstanding, the intention seems pretty clear, that the armor offers protection against hard vacuum.
Furthermore, in the opening segment of A New Hope,
when the stormtroopers board Princess Leia's starship, notice that the Rebels do not have fully automatic fire blasters. My interpretation is that the armor incurs a minimum charge to penetrate effectively, which imposes a cost on all weapons against the Empire. This cost would be significant galaxy wide, even with only partially effective armor. Those equipping the Tantive IV obviously could not afford better sidearms for the crew, than those which could deliver a charge sufficient to penetrate the armor effectively in at best semiautomatic fire mode.
Wankage at the Wookieepedia, on stormtrooper armor
and the Rebel sidearm
, supports my wankings here.