Looking at the picture these are starting to crystalize and are nitrate dice. Clean them with a coton ball or swab and water (do not imerse) and pat dry before giving them a "clear coat". Use nail polish or laquer. This will slow further degradation and make the dice safe to handle. Keep your dice away from heat and sunlight. Heat and ultraviolet will degrade both the older nitrate plactics and the newer acetate plastics. The vinegar smell comes from the newer acetate dice and yes degrading acetate dice are contagious becase of the acetic acid they give off. They should be cleaned with water and put in QUARENTINE from your other dice. And again keep away from heat. A clear coat will not help them. None of the older or present day plastics are forever and the polymers that make them up will eventualy unravel, but if you keep them away from ultraviolet light and heat you will dramaticaly extend thier lifespan. Rub faded logos with a white or yelow crayon as it makes them easy to read but does not actualy alter the dice.