I absolutely agree with you on Vicious not being fleshed-out. His character is probably the only thing I still can't figure out in Cowboy Bebop. When he was with Gren, it seemed like Vicious was just always an asshole, only saving Gren's life just so he can use him later, while Gren saw him as a real friend and war buddy. Spike, on the other hand, the 2 are depicted to have been good friends in his flashback when he was in the Red Dragon syndicate.
It doesn't look at all like Vicious has some kind of dark motive at this point, he seemed to truly found Spike to be a friend. So the question is: When did Vicious start becoming a villain? He knew Gren way before Spike, so he always had sinister motives, but it's implied he wanted to kill Spike only after he found out about Julia's affair. I'm so confused with his entire character, I'm having trouble figuring out what to write. Cowboy Bebop has A LOT of symbolism and there are some elements to characters that you really need to look closely to understand, but no matter how much I do it, Vicious still comes out as an uninteresting character. Julia, while I think she too is underdeveloped, I think Spike's flashbacks and own goals do a good enough job of bringing out what kind of person she is, and I don't see it as big of a problem as Vicious
Still, I thought Cowboy Bebop was amazing. The characters that are developed, whether you learn it through the story or through the subtext, really are the best parts of the show. It may not be my favorite anime, but Spike Spiegel is definitely one of, if not my favorite anime character of all time