The vacuum suit is one of the results of Mondain and Minax meddling with the stream of time. A technological item far beyond the abilities of ancient Sosaria, it is one of many anachronisms appearing in the lands during that time.
The vacuum suit is essential for travelling into outer space, as without it, death is absolutely certain. It essentially is an environmental suit, specifically designed to hold off the inherent danger of traveling in a vacuum. The suit can be bypassed if either reflect armour or the mighty power armour are available; both of these suits provide adequate protection from vacuum conditions, in addition to being extremely well-armored.
This armour was not to be seen after Ultima II.