This page contains information regarding Star Trek: Lower Decks, and thus may contain spoilers.
Due to the duplicative nature of this shapeshifting species, as of the latter half of the 23rd century, the Vendorian homeworld was under quarantine, and few people had ever encountered them. Per the terms of the treaty between the Federation and the Romulan Empire, any use of Vendorian spies was considered a violation of the treaty and hence an act of war. For these reasons. (TAS: "The Survivor")
In 2380, a Vendorian was impersonating an Andorian in a bar within the Andorian district of Tulgana IV. Andorians had the individual cornered, but they escaped due to the intervention of Brad Boimler, who thought the Vendorian was actually in danger. (LD: "Envoys")
Vendorians had the ability to rearrange their atoms/molecular structure into any shape or identity possessing the same general size and mass, such as an examination table, a deflector shield, or even a Human or Andorian. In spite of these abilities, they were not impervious to the effects of orientine acid. (TAS: "The Survivor")
When taking Human form, a Vendorian would emergence from the transition with the Human's arms crossed over their chest.
Vendorians also had the ability to render some humanoids, particularly Humans, unconscious by touch alone.
As it was the way the Vendorians, the longer they stayed in another's form, the more they take on its memory, emotion, attitudes. 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.
Due to their shapeshifting abilities, allowing them to be masters of deception, and such a practice was often their way of life, making outside interaction from others off limits.
The Vendorian culture place considerable importance on the value of each individual. Those who were non-producers, were considered useless, and deemed as an outcast, fit only to do those tasks of the lowest order, such as being assigned to care for the injured and nurse them back to health.
According to the Vendorian posing as Carter Winston, they possessed advanced medical care, capable of repairing, or attempting to put an injured Human together though a series of procedures, including surgery, skin graft, regrowth of bone. (TAS: "The Survivor")
The same adaptation (as well as the start of the novelization of "The Lorelei Signal", 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.