First question is how big and heavy are the steel letters? If they are actually steel to get that rusty patina you need to be carfeul on mounting. In that case stud mount is the way to go. Over time the rust will push the lettering off if not mechanically fastened.
Otherwise, can you cut out the vinyl where the steel lettering goes? That would allow you to VHB and Lexel the steel directly to the poly metal.
I agree with GaSouthpaw though. Overbuild it to last two lifetimes. Nothing worse than call backs.