Turns out they're some type of advanced beings that can be used as power plants.
So many stories, so little time.

I'm guessing a little bit of both

One thing I've always been confused about:
When they say "plants," do they mean like flowers and trees, or like nuclear power plants?
Can't stop the signal.

-Why did Rem and the others create human/plant hybrids in the first place?
Unless I'm mistaken they're not hybrids, Vash and Knives were two Plants who were woken up. The crew didn't create Plants, some other human group did at some point before the ships departed for a planet to colonize.
-What's the explanation behind Vash and Knives' superpowers? Things such as them stopping those giant light bulbs from blowing up, turning their arms into nuclear weapons of mass destruction with whatever was in those pistols, and telepathy?
They're Plants. The giant lightbulb powerplants are also Plants (in non-human form). Knives spent a long time creating the two guns specifically so he and Vash could be weapons of mass destruction.
"Need" is a man-made concept. As is "should", "must", "value" and "importance".