Back when Justice League was still in early stages of production, we found out that Willem Dafoe would have made his debut as the Atlantean Vulko in the movie. Dafoe was nowhere to be found in JL, but Jason Momoa talks about the scene that could have been.
Talking to Gamespot, Momoa explains:
"There's a place where [Aquaman] goes down to, and it got cut out of the movie. He knows spots where he can go and he can see these statues, the remnants of [Atlantis]. There's Vulko—obviously we'll have way more scenes with Vulko, Willem Dafoe–in Aquaman. There just wasn't enough time in this movie."
In the film, the only part of Atlantis we really got to see was the chamber where they kept the motherbox, and only Amber Heard's Mera was able to talk to Aquaman. As it turns out, Vulko will act as some sort of mentor to Arthur Curry and he has something to do with his backstory.
"Vulko's his connection to Atlantis. I think what Zack [Snyder] and I did, we were kind of trying to establish that he was taken down there as a boy, and he was an outcast, he was a half-breed, and he was built up as a young boy, because he was fed all these ideas by Vulko—that he was the rightful king. And he gets down there, and he's a half breed, he's impure, and I'm just made to feel like I'm this disease. So after that, I was like, ‘fuck you, fuck you, I'm on my own.'"
With Warner Bros said to be adamant in keeping the runtime of Justice League close to 2 hours, it would make sense that they would cut any scenes that didn't have anything to do with the main plot. Vulko is probably there for adding meat to Aquaman's backstory; even Cyborg and Flash had their origins left out.
For now, we'll just have to wait for Vulko to officially appear in the Aquaman solo movie in 2018, but maybe we can also catch him in some extended version of deleted scene in the Justice League DVD.
Catch Justice League in theaters now.