The Vendorians were a cephalopodic shapeshifting non-humanoid species that had perfected the techniques of cellular metamorphosis, and were capable of rearranging their own cellular structures to imitate any person or object of similar size and mass. While Vendorians could assume many shapes and identities, the longer they remained in one form the more characteristics of that form they would assume. This was particularly true when a Vendorian assumed a humanoid form. In these instances, the Vendorian absorbed memories and personality traits of an individual through touch. These memories and traits became more ingrained the longer or more frequently the Vendorian assumed the form; if maintained long enough, the absorbed traits eventually became part of the Vendorian's own personality.
As they demonstrated in their ultimately unsuccessful efforts to save the life of trader and philanthropist Carter Winston, Vendorians were excellent surgeons – due in no small part to having six tentacles in their true forms – and likewise possessed knowledge of other branches of medicine.
Unfortunately for communities of worlds, the Vendorians practice deception as a way of life and have been deemed psychologically unfit to participate in such communities on those grounds. Indeed, due to the Vendorians's deceitful way of life, the Federation imposed a quarantine on their homeworld, Vendor, declaring it off-limits to travelers.
Under the terms of the treaty between the Federation and Romulan Empire, use of Vendorian spies (or, indeed, any deception either side practiced upon the other to lure that other into and/or across the border between the two governments) was considered a violation of the treaty and hence an act of war. (TAS: "The Survivor")