Up to now I believe PVC is always opaque. Or is it very thin? Or is it polyvinyl acetate rather?
A hot glue gun
would be my very first choicehttps://en.wikipedia.org/wiki/Hot-melt_adhesive
Very strong joint on PET and PVC, waterproof and reliable, rather clear if not too thick. As applied with a gun, a nice result depends on the operator. Maybe a machine can apply it better.
I don't like silicone glues on polymers. They adhere naturally on ceramics, not on polymers nor metals, where only a primer gives some success chances. Too unreliable in my experience.Dissolving PVC makes excellent glues
, and I believe PET too. Alas, the solvents can be nasty, like vinyl monomer for PVC. Finding an acceptable solvent would lead to success. Check the "material compatibility list" for PVC and PET, where you see a strongly incompatible solvent which isn't too harmful, it's a candidate. If your material is polyvinyl acetate instead, this gets easier.
So-called polyvinyl acetate glues are candidates on PVChttps://en.wikipedia.org/wiki/Polyvinyl_acetate
In gluing, the main condition of success is the preparation of the surfaces, mechanically and chemically.
. Far better than glueing where possible. Hot glue guns exist specifically for PVC, with a feed of PVC wire. Other apparatus exists, with hot air, light, RF power, vibrations, and so on.