Well according to the episode, it seems as though the only spirit strong and powerful enough to rip Raava's essence out of the Avatar Spirit is Vaatu himself. I do not believe that having the Harmonic Convergence really makes a difference, unless if you want to argue the fact that Vaatu was freed during Harmonic Convergence. In the episode "Beginnings Part 1" it was noticed that a dark spirit had the capability of corrupting the Avatar spirit. I'm not 100% certain that it could rip out Raava's essence but it could destroy the Avatar spirit severely breaking the connection to the past lives. As we know from the "Beginnings Episodes," the two most powerful spirits are the spirits of Raava and Vaatu and it is understood that other spirits were terrified at the warfare between those two spirits. So basically, I conclude that Harmonic Convergence or not, if there is a battle between Vaatu and Raava (or the Avatar), Vaatu has the capability of separating his counterpart from the Avatar spirit.

According to the episode, it seems as though the only spirit strong and powerful enough to rip Raava's essence out of the Avatar Spirit is Vaatu himself. I do not believe that having the Harmonic Convergence really makes a difference, unless you want to argue the fact that Vaatu was freed during Harmonic Convergence.

In the episode "Beginnings Part 1" it was noticed that a dark spirit had the capability of corrupting the Avatar spirit. I'm not 100% certain that it could rip out Raava's essence but it could destroy the Avatar spirit severely breaking the connection to the past lives.

As we know from the "Beginnings Episodes" the two most powerful spirits are the spirits of Raava and Vaatu and it is understood that other spirits were terrified at the warfare between those two .

So basically, I conclude that Harmonic Convergence or not, if there is a battle between Vaatu and Raava (or the Avatar) then Vaatu has the capability of separating his counterpart from the Avatar spirit.