Seeing the face of Adrian Chase under the mask of Vigilante would definitely be a surprise to Oliver and the gang, and likely would shock most fans upon reveal as well.
While the typical Arrow-verse M.O. would be to say “this is Earth-2 Adrian,” it’s also worth mentioning that the Adrian Chase of the comics has a brother, Dorian Chase.
If Vigilante was Dorian, it was pretty messed up for Prometheus to toss his own brother off a building!
In either case, this Vigilante identity would definitely be an antagonist tied to Oliver, as that seems who he was most concerned with before he disappeared.