Your dog has a full thinkness laceration and your vet refused to stitch it. Did he have a good reason for it? If he didn't have one, I would have found a vet that would stitch it.
One of my previous dogs had a cut on the inside of the ear flap and the vet didn't want to stitch it because she thought it was too small of a lac to be worth sedating the dog to do and was also worried about potential infection. I put my foot down and said he didn't need to be sedated and I wasn't about to let his ear get all puckery from a badly healed wound. She settled for injecting local lidocaine, stitching it, and sending me home with topical antibiotic creme. It healed perfectly.
It's possible your vet was thinking the same thing. Too bad it's too late to stitch now.