Vapor steam isn't really that different from its higher pressure two pipe cousin. In fact most two pipe systems in houses are actually two pipe vapor systems. In a vapor system the pressure is either just below atmospheric, at atmospheric, or just slightly above atmospheric. Thats where the term vapor comes from. The pipes are significantly larger than a system that was designed to run say at 5 pounds, or even 2 pounds for that matter. The reason for this is there can be little to no friction loss allowed as there really isn't much pressure on the steam. Remember it is vapor. Its just waffling up those huge pipes. There is no vacuum pump on a vapor system. This link is a good description from a 1918 heating and ventilating magazine of what vapor heating is. Each system had its own style of ''devices'' as they had to work around each others patents. Thats part of the fun today. Just knowing what these devices do or did, can either make or break a systems operation if touched by unknowing hands.