Stream of consciousness as I read thru, then a conclusion at the bottom.
Fluff: Seems like this is made for Ravnica.
Symbiont Speech: Totem barbarians Spirit Seeker get speak to animals, but only as a ritual so it takes 10 minutes, plus that doesn't allow speaking with plants. Having this at-will makes even the speak with animals part much more useful. This needs a balancing weakness.
Symbiotic Rage: And this is it - a lot weaker than Bear totem, and will often overlap with the resistances you get just by raging.
Vines of the Symbiont: Not bad. Usually the 6th level is defensive or out-of-combat, but this is fine. It doesn't mention what to use as your spell casting ability score. I'd make it Strength just to keep the value.
Hey, no subclass ability at 10th?
Greater Symbiont Vines: Not bad, a bit weaker than other subclasses.
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Conclusion: Not overpowered, if anything a bit behind the curve, but flavorful. Missing 10th level ability and what ability score to use for Vines of the Symbiont.
Personally, I'd make make some changes.
Symbiotic Rage: Clarify that the resistance is to the first damage type that you aren't already resistant to. So it's something besides the B/P/S all barbarians get. Also while raging the barbarian counts as the most advantageous of humanoid, plant, or beast (for the target of spells and effects).
Vines of the Symbiont: use Strength and have no components (so you don't need a free hand, it's like a plant tentacle whipping out)
Add in something at 10th.
Greater Symbiont Vines: Change to explicitly count as a weapon attack, and a bonus action to cast. That means it gets rage damage and also that it qualifies for Reckless Attack. I was thinking about replacing attacks in an Attack action, but adding in several already damage-scaled-by-level is pretty powerful. This allows it and another action, including Attack.