I use Oralite for most of my fire helmet decals, but I thermally print it and then laminate it. You should be fine printing with solvent ink, but laminate it for sure. Just to CYA, make sure to tell your customer that you cannot guarantee any vinyl that is used on a fire helmet in live fire conditions, and also by applying that decal to the helmet he may void the manufacturer's warranty and/or its compliance with NFPA standards.