Else, I'm so sorry about your cat's addiction. It's tough to get over. Give him all the love and understanding you can. Since he/she isn't able, make sure you throw away directly any plastic you find. I know it's tough love, but ya gotta do it.
Now on a serious side: Your cat may have a blockage from the plastic. That's why he's throwing up. Any food that goes down may not be getting through. Call your vet. Please. Puss needs your help.
Hugs and purrs
Bren