I think it's pretty easy for the average fan to accept Vic in the Justice League even if they know him from the Teen Titans. Even if people don't know post-Flashpoint continuity where he was never on the Titans team. Characters are occasionally allowed to grow up and move on. In the animated movie Justice League: Doom they officially welcomed him to the team and I think they even had an episode of Teen Titans Go! where the League wanted to recruit him (which of course drove Robin crazy that they didn't want him). He was on the Teen Titans but he's grown up. Similar to how Robin was a sidekick but went on to be his own man with his own city to protect.
I was at the store a few months ago and a guy saw my Cyborg tattoo. He said "I used to love the Teen Titans cartoon!" When I told him he was gonna be in the Justice League movie he replied "oh he graduated."
So I've never felt like people will be confused or angered that Vic is now on the League. They'll just think he's grown up and made the natural progression to the big Leagues.