It would be as deadly as today's sniper rifle - i.e. one good hit = death. The only difference is that rifle works at long range and is mostly useless in melee and your weapon would be the opposite.
And just like sniper rifle it would be absolutely useless in warfare, rather than assassination. Even more useless than a rifle, actually, considering difference mentioned. When your position is stormed by 100 soldiers, you'll hardly have chance and time to approach and slash each of them with your vorpalfreeze sword, while rifle would allow you to have at least some kills while opposing force is closing up.
That was true for initial "50C reduction" version of question - we're 60%+ water and our usual body temperature is around 36C, so it guaranteed cooling of water inside body below its freezing point. You could just stick your sword pretty much anywhere and have affected part literally implode and rupture almost every every cell of affected area. Even hitting just limbs could be deadly thanks to great shock and clogged blood vessels.
Your newer "15C reduction" version is much less deadly as it now requires precise hit into vital spot or cold enough environment to make at least outer layers of body cold enough to still reach water freezing point with "help" of your sword.