What is the chemical reaction between coins (copper and nickel) and polyvinyl chloride?
Far, far too broad a question. I don't know what is going on in your situation.
I can, perhaps, clear up some misconceptions you may have. PVC is NOT commonly used in sandwich bags. PVDC is a close relative and used to be common as "saran wrap". But most sandwich bags are made out of polyethylene (PE). Chemically both PVC and PVDC degrade due to weathering and other things. Dehydrohalogenation is the reaction of organic chlorine compounds (both PVC and PVDC) which can be expressed in the partial formula C-HC-Cl → C=C + HCl (this is a crude simplification). Note that this reaction generates HCl, also known as hydrochloric acid (or muriatic acid). HCl is a powerful acid, attacking most metals. If you live by the ocean or in a desert chloride levels in the air (dust or mist) can be quite high and can accelerate corrosion dramatically. Coins are designed to be inert. Placed clean and dry into a sealed PE bag, they should not corrode. But. Whenever two different metals (or metal alloys) come into contact, a electrical potential is created. A random bag of coins will be sorta like a bag of little batteries. That is, you shouldn't be surprised if they react (discharge), especially in the presence of moisture. Neither copper nor nickel form green oxides, but both do form green chlorides and green carbonates. You need a source of chlorine for the chlorides, but the carbonates might form when simply exposed to air and moisture when a electrical potential exists. Sweat, sweaty hands, are potentially one source of chloride/chlorine, but as I stated up front I am not familiar with your situation, I carry (mixed) coins in my pocket for days at a time without any green forming.