Well, one possibility could be that they decide to have a small, intimate wedding. Maybe they choose a beautiful outdoor location like a secluded beach or a charming garden. Their friends from the supernatural world would all be there, with Damon making some snarky but heartfelt toast about how he never thought he'd see this day. And Caroline would look stunning in a simple yet elegant white dress.
Perhaps in the fanfiction, they get married as a way to protect each other. With all the threats in their world, being married might give them certain legal or magical advantages. Alaric could be the one to plan the whole wedding secretly, and when he reveals it to Caroline, she's initially shocked but then overjoyed. They would exchange vows that are not only about love but also about loyalty and standing by each other through all the supernatural chaos.