The Vesbians were genetically altered Humans native to the planet Vesbius in the Omega sector. Their population numbered to about 20,000 people. They were known as master horticulturalists and biologists, with their pharmaceuticals being an enormous boon to the Federation, even though they were not members of it. One example of their work was the discovery and development of the Rigelian fever vaccine. They were also know for their hospitality and trade agreements, with a local beverage known as Vesbian ale being well known in the Omega sector. They also had their own dance called the Vesbius Tawla, which took some getting used to in order to learn. But while they were known for their hospitality, some Vesbians were suspicious and even hostile towards aliens, like Vulcans. They were governed by a planetary council, which consisted of 15 people.
The Vesbians were once fully human and their world was once part of the Federation. The original colonists consisted of retired Starfleet officers, mostly science officers and many biologists. Fifteen years before James T. Kirks third year of commanding the USS Enterprise, they separated from the Federation due to refusal to uphold and maintain Article 17 of the Official Federation Homestead act, which prohibited any manipulation of the human genome. The original colonists had a hard time adapting to the environment due to increasing allergies, with symptoms growing worse over time. This led to the first generation of Vesbius-born children to die out. In order to stay, they modified themselves with elements of the native Vesbian environment, literally becoming one with the planet's biosphere. The process was irreversible, but they were able to be in tune with the environment, feeling what the planet feels. They then planted a tree called the Birthing Tree that served as a memorial for the first generation of children that died.
The altering process allowed the natives to be more healthy on their own world. They were able to leave Vesbius, but only for about three to four weeks or they would die, with their bodies rejecting their implanted genetics. They were able to take medication to ease the pain but it would not last very long. The native born would show more weakening symptoms quicker than the original colonists or recent immigrants.
The adding of the genetics eventually led to the formation of the terrorist group known as the Exos. They believed in their own superiority and hated the inferiority of others that were not like them. They executed their plans by any means necessary, even killing some of their own people to achieve their goals.
The Enterprise's crew tried to warn the Vesbians of an asteroid that was heading for their planet, but most were not willing to leave for the reasons described above. Instead they made deep shelters underground to hide in, but the crew were worried about the events that would happen when the asteroid did hit. It also did not help when the Exos blew up one of the tunnels, killing about 200 people and injuring even more. But with the help of the Enterprise, along with the help of the Horta, the asteroid was broken up enough so the planet's surface did not become completely destroyed and the plans of the Exos were stopped. (TOS novel: Devil's Bargain)