I am making my backup character for a campaign coming up just in case, and the character I’m making is Vash The Stampede from the anime/manga Trigun. I’ve got most of everything I need for him done except for a very specific thing: What race would he be? If you don’t know Vash is essentially a plant that is a human form. His aging process is slowed considerably, very intelligent from birth, with a slight regeneration factor and excellent reflexes. Plus it’s assumed he can use solar energy to keep him going. Obviously home brew is possible but what class could I use if homebrew is not available?

I would ignore the label "Plant".  Japanese translations means subtle overtones are lost when it comes to things like this.   He is a supernatural human that ages slowly and does a bit of shapeshifting.

I might consider a Simic, as they have some shapeshifting, but their life span is shorter.   The other option is to be some version of an Elf.  Perhaps a Wood elf to represent the "plant" nature.