As per the title: Thin Film integrated into the VrayMtl, perhaps most logically as a checkbox and parameters under the Coat Parameters introduced with V-Ray 5.
I understand that the effect can be made with an OSL shader or a Siger texture in the reflection slot but these don’t work with V-Ray GPU.
Beyond bubbles and oil slicks, many materials can benefit from thin film iridescence - for example, leather and plastics.
Cheers!