After you glue them, a good vinyl rejuvenation product, something a little stronger than Armor-All should be used. Over time some of the chemicals evaporate out or are degraded making plastics brittle, and you want to add as much as possible back. I don't know what's the best product, I've just been trying things.
I have been painting old plastics to stabilize them. I wash them in hot soapy water then wipe the surface with alcohol to remove oils. I put on a good plastic primer then an appropriate paint. That seems to really help prevent future damage, and it hides repairs.