In season 5, Arrow introduced DC Comics anti-hero Vigilante, but made a surprising move when it changed his identity to a different character. In the comic books, Vigilante’s civilian alter ego is Adrian Chase, a district attorney who decides to take the fight to the streets of New York City after his family is murdered by criminals. After regularly engaging in violence against organized crime, Vigilante became deeply troubled by the bloodshed that became a part of his daily life. In the 1980s, the character was popular enough to headline his own comic book series.
In the Arrowverse, Vigilante is in fact a cop named Vincent Sobel, rather than Adrian Chase. It was revealed in season 6 that Vincent was the supposedly deceased partner and boyfriend of Team Arrow’s newest Black Canary, Dinah Drake (Juliana Harkavy). Vincent was believed to have been killed by a gangster, but as it turned out, he was saved by the particle accelerator explosion that occurred in the series premiere of The Flash. After becoming a metahuman with regenerative capabilities, Vincent went on a ruthless crusade against organized crime as Vigilante and became an enemy of Green Arrow (Stephen Amell) and his allies.
As for why the decision was made to make Vigilante a new character rather than Adrian Chase, the reason can be explained by the story for season 5. Both Adrian Chase (Josh Segarra) and Vigilante were introduced at the beginning of season 5. Adrian at first was presented to be much like his comic book counterpart, as he too was a district attorney determined to clean up the streets. This, combined with the timing of their introductions, made him a prime suspect (to the fans) for being the Vigilante. This change helped Arrow season 5 hide its biggest twist, which is that Adrian Chase was actually Prometheus.
Since Prometheus was the main villain of the season (with Vigilante being more of a secondary antagonist), Prometheus’ identity was of course the bigger mystery. Considering that Adrian Chase had a strong presence on the show at the time, it would make sense to suspect him, but his name being tied to Vigilante in DC Comics helped cast suspicion off him. Plus, Arrow dropped other clues that Chase might be Vigilante. For these reasons, it came as a big surprise when Prometheus unmasked for the first time. Stephen Amell and others have said that Arrow intentionally used the comics to make sure the reveal was unexpected [via EW].
The deception worked and helped Arrow deliver one of its most memorable villains in Prometheus, a person who hated Oliver so much that he’d rather torture him mentally than outright kill him. Following the conclusion of its Prometheus story, Arrow created another interesting character when it turned to its real Vigilante, Vincent Sobel, and used him and his alliance with Diaz (Kirk Acevedo) to give Dinah a compelling arc of her own.