The Module A is an addition that can be used to upgrade a house. The Module A is known as the module. Someday modules will give perks to a house, such as processing tools, vendors, a stable, and so on. Module A is available in a tier 1 - tier 3 house. The Vendor Module seems to open up a second door used by a vendor. It seems to be made for others to come to that door, open it, and buy the wares of the vendor inside. Most likely a way for players to have a shop.
Photo thanks to the House Design thread in the beta forums.