delaminated ceiling

Is it just the thin vinyl covering, like a wallpaper or is it the vinyl and backing material such as a luan? If it's just the thin part like a wallpaper and you're sure the leak issue is solved, I'd do the same as if I had a bubble in some wallpaper. A sharp single edge blade, make an X so that you can fold the sections back that aren't adhered. Use a good wallpaper adhesive and apply to the vinyl and use one of those plastic smoothing tools - maybe 4" or so and gently smooth out the bubbles. Take a slightly damp cloth and wipe any adhesive that squeezes out and let it dry. You may still see the cuts from the razor blade, but the adhesive should eliminate the bubble. If it's more than just what I've described, I'd leave well enough alone. If it's into the backing/luan/whatever I wouldn't mess with it.