There seem to be a few companies in the UK who advertise Vinyl decals for curtain siders. One manufacturer (metalmark) refers to issues with plasticiser in the curtain fabric which requires more specialised adhesives.

Some links:

https://www.sapphiregraphics.co.uk/gb-en/graphics/curtainsiders.php

https://www.channelcommercials.co.uk/signs-and-graphics/vehicle-graphics/truck-curtain-livery/

https://www.metamark.co.uk/support/application-guidelines/application-to-plastics/

Paul
Costock, UK
Plus Four 75th - Furka Rouge - Pip
Disco 5
Teddy - 17h1 Irish Draught cross