It may just take some time. I don't think it's that they don't remember each other, or have suddenly taken a dislike for one another. They've been through a traumatic experience, and the vet smell is a strong reminder of that. Recall how strong a sense of smell a cat has, and how much they rely on that. I definitely wouldn't do scent swapping for awhile
That's the last thing I would do. If you are able to wipe down their fur or anything like that it might help. Otherwise they will tell you when they are ready. I'd use most of the other reintroduction principals though. Try to set up positive interactions with each other, try to end it on high notes, and try not to let hostility happen.
I've never personally had it last more than a few days, but a friend of mine had it last for a couple of months with one of her cats. I think that is very unusual though.