Roasted garlic, roast the peppers , roast the onion, I'd probably use more garlic.
I'd use a milder vinegar if you use vinegar. Something like rice, white wine or apple cider vinegar. I'd use a sweetener. Maybe a little honey or agave nectar. Shoot, you could use sugar but I'd stay away from white granulated. If using sugar use brown.
Oh and when I roast things like peppers, I leave the char on. Some people will take the skin off the peppers but I don't.
You could definitely use those red bells but I use dulcettas. Sweeter and better tasting in my opinion.
Boss man says lemon but I prefer lime.