Do vinyl decals need Mod Podge before epoxy, and how long should it dry?

Yes. Epoxy does not adhere directly to vinyl, so every vinyl sticker or decal has to be sealed with Mod Podge first, even when there is bare wood around and between the decals. Without it the epoxy can't bond over the vinyl and you risk fish-eyes or delamination.

The order of work: 1. Prep the surface. Water-based stains and latex or water-based paints are fine; epoxy can go over paint after about 24 hours. Oil-based paint needs 3 coats of oil-based polyurethane first. 2. Apply the decal and brush Mod Podge over it. Mod Podge dries clear, so an uneven coat on the decal itself won't show. Where it overlaps onto bare wood it can be noticeable on some darker woods, so either apply the Mod Podge before adhering the decal, or tape off the outline with painter's tape and remove the tape once the Mod Podge dries. 3. Let the Mod Podge dry for at least 24 hours. 4. Apply the seal coat and flood coat of UltraClear Bar & Table Top Epoxy as normal.

Decals go on before the seal coat, not between coats. The vinyl needs surrounding wood for the epoxy to grip, so smaller decals on a larger wood surface work fine, but a single large decal covering the entire surface can't be coated. Sealed this way, stickers and decals have been going under our epoxy for over 10 years without issues.

Mod Podge: https://www.bestbartopepoxy.com/collections/tools-supplies/products/mod-podge