I'll come out and say it: I seriously doubt Adrian Chase is Vigilante in Arrowverse. It's just too obvious.
The way things are going, there's obviously going to be some more buildup before a dramatic reveal of Vigilante's identity, and if it's Chase, the viewers not in the know about his comic counterpart might be shocked, but the rest of us will be like: "Yeah. It's Chase. We figured it out before this season premiered. Why all the dramatic buildup for something so obvious?"
So this is my take on who it might really be.
DISCLAIMER: I don't own Arrow.
WORD COUNT: 620
"Well, hello there. What is the mighty DA of Star City doing at my door?"
"Thinking that he has been spending more time than he should be at the office and not as much time as he should be with his brother, and feeling the need to rectify that." One hand raised to display an offering. "I've got the drinks, you've got the flatscreen, and there's a game on tonight."
Dorian Chase and grinned and gave his brother a hug before allowing Adrian to enter the apartment. "Sounds like a solid plan. I was just finishing up cooking some lasagna. I was going to box up half of it and save it for later, but I suppose I could let you have it instead."
Adrian's eyes lit up. "Mom's recipe?"
"Would I ever use any other? Go ahead and turn the game on; I'll only be a couple minutes." Dorian retreated back to the kitchen while Adrian went over to the couch set in front of the TV, setting the six-pack on the table. "So how are things going at work?" Dorian asked him.
"Ah, same as usual. Vigilantes being a pain in my ass."
Dorian chuckled. "How many DAs can say that's normal?"
Adrian shrugged. "Well, at least the Green Arrow's group is trying to play by the rules, providing evidence when we need it so we can actually convict the sons of bitches they drag to our doorstep. But there's another one who'd rather just kill them all than let them face justice."
Dorian was quiet for a minute. "Some would argue that that is justice."
The other brother sighed. "I won't deny that there are scumbags out there that deserve to die for their crimes. But that's for a court of law to decide, not some masked man on the streets who doesn't care if innocent people get caught in the crossfire." He was ripping the plastic bindings off the beer cans as he spoke, and accidentally knocked one off the table. "Whoops," he muttered as it rolled away and bumped into a door that was slightly ajar. He stood, walked over, and picked it up, but froze when something caught his eye.
He hadn't been to this apartment often; Dorian had only moved out of his old one a couple months ago. And the few times he'd visited, Adrian hadn't thought to ask his brother what laid beyond the always-closed door. This was his first time seeing into this room, and he wasn't liking what he saw.
He pushed the door open even further and stared in horror at the racks of heavy-duty firearms. From small pistols to massive machine guns, they lined an entire wall from one end to another. When he finally tore his eyes away from them, he found himself faced with a corkboard covered in newspaper clippings. Stepping closer, his eyes picked out headlines and pictures, all relating to crime in Star City.
"Adrian!" He whipped back around to see his little brother standing in the doorway, an impossible mix of fear and outrage blazing on his face and in his voice. "How could you- You shouldn't- You weren't supposed to see this!"
Then came the worst of it. Adrian spotted something that he'd missed upon entering the room. Tucked away in one corner, partially hidden from view by another gun rack, was a mannequin holding a mostly black costume with a red-orange visor. Adrian knew exactly which criminal fit that description.
His brother was Vigilante.
"Dorian," he gasped brokenly, "What the hell have you done?"
THE END
Assuming this version of Chase isn't Vigilante, and he's just a Red Herring to distract us, there still has to be a connection. Because if that was his only contribution to the season's plotline (which would go right over the heads of unsavvy viewers, anyway), he wouldn't be billed as one of the main characters. But he is, so he has to be playing a more important role and getting more character development in later episodes.
There are many DC characters who go by the alias 'Vigilante', and of those, there are three that are directly connected to the Adrian Chase incarnation: his two friends Alan Welles and Dave Winston and his brother, Dorian Chase. So those three are the most likely suspects, in my opinion, with Dorian obviously being at the top.
On a side note: I also doubt Malcolm Merlyn is Prometheus, as some have suggested. It was already announced months ago that he and the Legion of Doom would be the Big Bads of Legends S2. The odds of him being a season Big Bad of Arrow once more AT THE SAME TIME is unlikely.
Last point: I have no intention of writing a sequel to this. This is just me throwing out a bit of speculation. If you want to build on this idea, write it yourself.