True, there's a pretty huge disconnect between how Vergil is portrayed in DMC1 and how he's portrayed in most other DMC media. American comic Nelo Angelo was much like his game persona (only this time with lines), but Gilver (Vergil in the first DMC novel) was a complete sociopath who deceived Dante and did horrible things to everyone Dante loved; DMC3 Vergil wasn't nearly as hateful against humans, but he simply didn't care what happened to them, and he betrayed Arkham; manga Vergil was closer to Gilver in nature, murdering people pretty casually; DmC Vergil was a riff on DMC3 Vergil, not hateful, but not honorable, either, with no problem going against his word.
One theory is that the only reason Nelo Angelo seemed honorable was because the only time we saw his behavior was in a direct fight with another warrior. Vergil might do dirt against non-warriors like Arkham, but we never see him approach a fellow warrior in a dirty way like, say, Phantom does with his ambushes, or Mundus does with his psychological warfare. He's not an honorable PERSON, but an honorable FIGHTER.
Another theory is that Nelo Angelo was brainwashed and stripped down to his most base mental components, and at Vergil's core, he's a good kid led wildly astray by pride and ambition. Of course he wouldn't be allowed either of those things as a servant of Mundus, so without them, he ironically starts to behave more decently than when he WASN'T serving a horrible demon emperor. Which is pretty sad.