Are plants vegan?

Symbiotic relationships =/= carnivorism.

Well, I just mean, are they dependent on us mostly, us being sea life, et al, or could they mostly get on just fine with some sunshine.

A cactus doesn’t need animals right?

Not a political commentary against veganism, here. I only eat plants…

Mushrooms have a vital symbiotic relationship with many types of plants, especially trees. The trees supply energy to the mushrooms, which return the favor by providing nitrates and minerals. Mushrooms are fungi, neither plant nor animal. Of course much of what mushrooms supply is from animals, but I suspect that nature would eventually adjust. There would initially be a large impact. Just think of the major role that pollinators play.

Well. Dead animals break down in the soil, and add certain nutrients. Some plants can probably do without dead animals, but some might die off eventually, if nothing other than plants existed.

Animals are very important to distribution of plant’s seeds, and pollens, as well…

A lot of plants are dependent on insects or birds or other pollinators for sexual reproduction.
Lots of plants depend on animals to scatter their seeds, the fruit can be eaten or a seed stick to fur. Plants that propagate by cuttings would be greatly aided by animals that carried a chunk around with them, e.g. a chunk of cactus stuck in some hair.