I've heard about this movie, and does anyone know what its all about?
Also yeah, the manga is leagues better than the anime. You get a better in-depth look and aspect into what Vash and his brother really are, as far as the whole "Plant" thing is concerned anyway.
Peace.
I'm assuming it will be something like Hellsing Ultimate and do a more faithful rendition of the manga, OR, it will be a continuation of the anime and have more closure. I have always prayed for a Trigun OVA that will follow the manga and cover the whole bit with the eye of Michael, this is doubtful however.