is the surface like this attached image, Iain?
they normally are, but “this is a very close up photo” zoomed in, so appears much flatter.
not all, but most are made from a thin metal sheet or grp that’s coated in a grained PVC coating, called plastisol which comes in about 60 colours and has a 25-year colour stable life. its repellant to many things and for this reason is used to clad factories, portacabins and so on.
it will take vinyl though and because of the natural vein-like grain on its surface, it acts like a bubble-free vinyl.
in short, your vinyl can be applied to it.
however, that’s “IF” it’s this type of surface coating.
As martyn correctly points out. some have a sort of low energy surface, which is normally just a cheaper type pvc coating that is again easy for them to be wiped clean.
I have been using this plastic-coated metal in 10ft x 49inch deep sheets, for around 25+ years now and we buy it in by the ton.