A prevalent theme is the exploration of their different worlds colliding. Vash has his own world of being a gunman with a mysterious past, and Meryl comes from a more normal background. Fanfictions often explore how they bridge that gap. For instance, how Meryl gets used to Vash's erratic lifestyle, and how Vash adapts to having someone like Meryl care about him in a different way than others.
Sure. You can try searching on platforms like Archive of Our Own. There are many talented writers who create interesting vash x meryl fanfictions. Some might focus on their adventures together, exploring how their relationship develops in different scenarios.
Vash doesn't die. The manga's storyline keeps him alive through all the challenges and adventures. The creators chose to give him a different fate than death.
Definitely not. Vash is a figment of someone's creative mind. It has no roots in true events. The entire concept is built from scratch for the purpose of providing an enjoyable and imaginative experience for viewers and readers.