Thanks for that, sIKE, I couldn't find it anywhere on this thread. Vine markers are far worse than I imagined, really fragile!
It seems like Pestilence, Gas Cloud, Plagued etc will only harm creatures (as per text).
So the above Poison effects harm Kralathor, Togorah, Vine Snapper, Raptor Vine etc
Poison cannot harm vine markers, Seedling Pods, Vine Tree, Samara Tree, Etherian Lifetree, Wall of Thorns, Tanglevine, Mana Flower
I guess this a necessary balance rule, like how former can be targeted by enchantments like Bear Strength and Rhino Hide. Maybe we can explain it as "the former are sentient plants"? It seems to me Poison can only harm plants with activation markers for some reason.
A necessary evil, I guess, as they made Vine MARKERS a conjuration (which also breaks another consistency in rules about markers).
I think they have a created a mess in consistency and intuitiveness here. Sad panda.