The show said that if both portals were opened then Vattu would brake free but if both portals were closed nothing would happen. What would have happened if one portal was opened and the other was closed?
If you have watched the Legend of Korra you will remember a certain episode in which Korra and Jinora venture into the Spirit World. Read below.....
Source is from the Avatar Wikia: Together with Jinora, Korra meditated into the Spirit World, where the two eventually found themselves separated. Jinora made her way to Wan Shi Tong's Library, where she found a book detailing that if both spirit portals were open when Harmonic Convergence occurs, spirit energy would be amplified, enabling Vaatu to free himself, starting the battle between good and evil anew
On the Avatar Wikia it states clearly that only if both portals are open during Harmonic Convergence, only then will Vaatu be able to escape from his prision also known as the Tree of Time. The reason....
Because of the amount of spiritual energy released by the both portals when they overlap making a dark purple beam of energy.
So in answering your question on 'What would happen if there is only one portal open during Harmonic Convergence'?, essentially nothing. Without both portals open there would not be enough....
- Spiritual Energy
- Spiritual Energy would not be amplified as much
- It would not be enough energy for such a spirit as Vaatu to escape although it could possibly happen.
If you need more information, it may be prudent to see the website avatar.wikia.com and learn more about Harmonic Convergence and Vaatu and Raava.