Any way to turn elmer's glue into vinyl or rubber like substance?
Elmer's Glue All and regular Mod Podge are the same thing (PVA/polyvinyl acetate glues). Notice the "vinyl" in the name for that type of glue. Regular Mod Podge has just been thinned with water (or perhaps acrylic medium) in order to make it thinner (so dry clearer) when being used for "decoupage." As for a "strong" coating, PVA glues (and acrylic paints) will be waterproof (or at least highly water-resistant) *once they're dry*, but will be a little soft compared to other adhesives like 2-part epoxy glues, clear "cements," and also polyurethanes and epoxy resins, etc. If they're left to soak continuously in a liquid for a long time, they'll get even softer, and will begin to loosen and peel off of their surfaces.
Elmer's glue isn't a "strong coating" because it comes off with water.