Vash's past is a somewhat mysterious one, and something that you don't learn about until well over half the series is played out. Thus, this info should be regarded as a huge spoiler section. Beware Millions Knives, and if that name means nothing to you, don't, under ANY circumstances, read this section.
I warned you. Anyway, first and foremost, Vash isn't quite... human in nature. He's ageless, one of "those who live outside of time." Y'see those huge lightbulb-looking things that are almost always next to the towns? Those are commonly known as plants, left over from Project Seeds, which is how all the people got onto that planet in the first place. The Earth was growing unliveable, so a group of colonists set out to find a new planet, one they could live on with their giant plants, making the areas liveable. As far as I can tell, whenever these plants get to a certain energy level, a child is generated, one who protects the colony from outside forces. They cannot die from old age, and always have the appearance of being in their 20's. In the manga, they die if they overexert themselves and use up their powers too much. Their hair will start to turn black toward the roots, and when it goes all black, the being will die. Vash's hair is going black at the roots at the beginning of Trigun Maximum, and appears to get progressively worse throughout. Knives' hair starts going dark at the beginning of Maximum 6. I _think_ Vash was born en route to the planet, along with his brother Millions Knives. Although they were small children, they grew at an abnormally fast rate, because again they weren't quite... human. They got to looking like normal ten-year-olds in about one year, and were much smarter than the adult crew members.
While on the colony ships, Vash met the most influential person of his life: Rem Saverem. Rem took it upon herself to sort of raise both Vash and Millions Knives, teaching them the values of human life, love, and peace. "The ticket to the future is always open." as the saying goes ^_^; Vash grew to love Rem, and took all her philosophies to heart, like the values of human life and the great dishonor of pain. Rem made Vash what he is today, period, end of sentence. The same cannot be said for Knives.
Unfortunately, since neither Vash nor his brother are quite... human, the crew treat them very poorly, abusing them and such. Well, not the entire crew, just one of the engineers named Steve who seemed to be a big pervert and very brash and unlikeable. He beat up Vash and Knives on at least one occasion apiece. This turns Knives very bitter and blind to Rem's teachings about the values of life, and makes him come to his OWN conclusions. Knives plotted and executed a very complex plan to blow up all the Seeds ships , saving only himself, Vash, and Rem (Rem only because Vash liked her, believe it or not the antagonist thinks very highly of the protagonist in this series) , so they could live by themselves on the planet with one of the seeds, and the planet would not be spoiled by human life, which would be like spreading a virus across our healthy Galaxy ^_^;. The plan was successful in that the core 5 members of Project Seeds, including the one that had beat up Vash and Knives, were all dead, but it failed in all the other aspects. When Rem saw that people were going to die and Project Seed would fail, she ran to correct the trajectory of the ships carrying the frozen colonists (most of the people participating in the project were in a sort of cryogenic freeze-type thing), leaving Vash and Knives in their escape pod and sacrificing her own life. But it is because of Rem that the planet has life on it today, so she did not die in vein. Rem's death was catostrophic in Vash's eyes.
When they were on the planet, Vash and Knives had a little disagreement. They wandered around in each other's company for quite some time, but Vash hated every minute of it due to Knives injustices to Rem and the human race compounded with the fact Knives continually talked about destroying them all. At one point, Vash even tried to kill Knives in his sleep by beating him to death with a rock, but Rem's words stopped him. When both had reached adulthood, they happened upon an abandoned SEED ship. Knives dissappeared inside while Vash waited outside for a year. When Knives finally emerged, he was wearing a new suit and also carried black and silver matching guns that he had created. The silver one was given to Vash along with instructions to eradicate the human race. In a panic, Vash shot Knives through the leg, grabbed the black gun that Knives had intended to keep, and ran off, leaving Knives. Years later, Vash happened upon Knives, and after a brief fight, Knives shot off Vash's arm to get the black gun back, thereby taking one of Vash's original Angel Arms (Knives kept the arm he shot off and gave it to Legato, but that's another story -_-;). Later, Vash had his missing appendage replaced by the machine gun arm, and thus he came about all three of his weapons (and the name of the series ^_^).
During the course of the series, Vash has set out to find his brother and bring about an understanding about the meaning of human life, and how it should be treated delecately. Along the way, we get to see him at his best (y'know, saving good-natured towns from small-time baddies, playing with the local children ), and his worst (when the diablo eyes show themselves, or when he takes a life against all his morals). At the end, we have the final showdown with Knives.
What does the future hold for Vash after Trigun? If all is right in the world, Love and Peace.