I know that they generate energy and they're used like power plants but there's also a person or creature of some sort inside. What are the plants exactly and how is it that Vash can communicate with them?
In the Trigun Universe, Plants are constructions, built by humans, to hold and house powerful beings that can create energy, food, and even water. Not much else is revealed about the beings themselves in either the manga or anime that gives away where they came from, how humans learned to harness their power, or what they truly are.
As for why Vash can communicate with them:
Both Vash and Knives are "products" of the plant. It is unknown if they were born by the plant on the ship, or how they came to be, as it is said that they were found near the plant itself. Vash and Knives both show many characteristics like the plant, and also many dissimilarities. For example, both show increased control over their powers, including being able to generate guns at will, and both have a far longer life-span than humans, while aging to the mid-30's look faster. They are also able to communicate with the plants, as Vash demonstrates on a few occasions. However, unlike plants, they do not need to be contained in a special environment to survive, and they are male, where as every plant form that can be seen is female.
This is in Wikipedia in Spanish and I just translated it for the question:
In the future, Earth has been overpopulated and natural resources and environment are in a state to disappear soon. In this situation they create the project "Seeds", in which, to ensure the survival of the species they sent into space a fleet of spacecrafts in which a great number of people are contained in suspended animation.
These ships used as feed system a technology called "Plants", a power generator housed in an entity whose core is almost pure energy and serves as a source of power to the ship.
In one of these ships, a small crew is responsible for guiding and find a planet that can accommodate the human race. One day, an anomaly leads to the reactor to discover there that not only the Plant has given birth two newborns, but unlike the rest of their species, they look more human and can survive outside the reactor.
From what I can tell, the plants were not created by humans, they were harnessed by humans as an energy source. The humans in this series do not seem to have a complete understanding of the plant entities. Vash can communicate with other plants, since he is a plant (or as the anime suggests a possible plant-human hybrid). Note that a crew member dies in the ship's engines just prior to the appearance of Vash and Knives.