The Vendorians were a non-humanoid species capable of shapeshifting into anything of the same general size and mass, and known for their practice of deceit as a way of life. For these reasons, their homeworld Vendor was quarantine and few people ever encountered them.
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.
The same adaptation (as well as the start of the novelization of TAS: "The Lorelei Signal", as published in the same book) establishes a history between the Vendorians and Romulans, stating that – for several years prior to 2269 – there was a period in which the Romulans frequently visited the Vendorian homeworld, interested in an alliance between the two species. The Vendorians kept rejecting the Romulan offer of such an alliance, as the Vendorian impostor of Carter Winston explained.
A further hint regarding Vendorian culture is that the Carter Winston imposter, who took on the task of caring for an injured Winston for a year, was considered a useless non-producer and an outcast, fit only to do those tasks of the lowest order.