When Korra was being forced into the Avatar State, I think it is possible Vaatu actually did appear. After all, he is currently a part of Raava, which means perhaps he managed to manifest just enough to appear to Korra when she was both weakened, and entering the avatar state.
Omnibender wrote: Really unlikely. Bryke have said in interviews that even though Vaatu will grow again from Raava, he will not be strong enough to do anything in several lifetimes.
This doesn't mean he can't at least manifest a small consiousness when the avatar is weakened, and channeling Raava's power. Don't forget, Raava was brought back quickly, and In fact, I think its entirely possible he was also influencing Korra's hallucinations, visions, and nightmares during the 3 years between Books 3 and 4.
They used the phrase "too weak to do anything," & then to "check back in 10,000 years." Anything. For 10,000 years. This was in direct response to a fan question, so if their intention was that Vaatu actually appeared to Korra, they would have specified the exception.
Though, if you ask me, they shouldn't have needed to say so, because it was clearly framed as a hallucination, did not happen since then, & Vaatu was not exposed to Deus Ex Jinora to bring him back prior to the 10,000 year mark.
Raava is clearly smarter than Vaatu. as he believed he could permenently kill her. allthough i supsect he could. jinora gathered up all the light in the world and revived Raava inside Vaatu. there is no proof that Vaatu is currently alive inside Raava it takes 10,000 years for them to revive. and Raava knowing this could easily just perge the darkness from her every few years. meaning that even in 10,000 years Vaatu will simply stay asleep.
the entire reason that Wan locked Vaatu up in the tree of time was to avoid the next harmonic convergence. meaning he was meant to stay in there the prison cell was only able to be opened by the avatar and he had no way of influencing things outside of that cell of his so for all we know Raava simply would rebuild his cell inside herself