Trigun never ever resolved or explained many things. However, I have complied different bits of evidence within the show pointing to exactly what in the world Vash is.
!~!CAUTION!~!
SPOILERS AHEAD
READ AT YOUR OWN RISK!
Okay so the spoilers aren't that big, but I felt I should say that anyway.
Vash was discovered on the Seed ship which was carying the plants along with his twin brother Knives. Thanks to Rem, the two were saved from the crew memebers who found them and wanted to kill them. The reason the crew wanted to kill Vash and Knives was because they had checked and checked and checked that area whenever they were awakened from hyper-sleep. (If you're not following me, go to the show explaination. That should answer any questions you might have.) So obviously the crew members believed that these two babies were aliens or invaders of some sort. However, Rem, being the kind and gentle person that she was, protected them from the crew memebers. Over the next year, Vash and Knives grew very quickly. By the end of that year, they looked as if they were at least 7 years old. So, here is the first question, why did Vash grow funny? That one I'm not too certain about, but I do have a theory. My theory is (and this might be long) that these two characters are "angels" or a Buddhist equivalent. Because, I think that the whole principle behind Vash and Knives is based in the Buddhist religion. That sounds a little weird, but the more I thought about it, the more it made sense. Vash is the exact opposite of Knives. He's pure of heart and kind to all, and he loves all. Knives is purely cruel, and hates all mankind. They are good and evil, black and white, the yin and the yang (good cannot exist without evil--a Buddhist belief).
Now we get into predetermined destiny. If you think about it, the reason they are there is to put mankind on that desert planet, and then make it green. I'm getting that info from the last episode, where Knives is standing in a green oasis. He apparently made that, because he tells Vash that they are going to make a green planet for themselves. So, getting back to that question, 'Why did Vash and Knives grow funny?', simply because they had to be old enough to take care of themselves when they crashed onto the planet. Because, it was meant to be that Knives would inadvertantly kill Rem when he plotted the ships to crash onto the desert planet, leaving them with no one to care for them--except themselves. Actually the whole idea that they're there to save and protect mankind basically covers any and all questions about Vash. That explains why he's there, why he loves every living creature, and why he will live forever.
Vash is a big loveable guy, who is kind and gentle--just like Rem. His favorite foods are doughnuts and salmon sandwhiches. He plays the fool to throw off baddies, and then surprises them with his high intelligence and gunmanship. He's an outstanding gunman, but he has to be so that he will never kill anyone; because of Rem's influence over him, he has vowed never to take the life of another living being. The only problem is that he's living for Rem, not because he wants to live. He is constantly thinking of what she would have done, or what she would have thought in certain situtations. It isn't until the end when he figures out that he wants to live for himself, and find his own words, instead of spouting someone else's beliefs. Also--as a side note--Vash is not in love with Meryl. Because, well, if you want to know, go to the Milly and Meryl page.
Love and Peace.