I doubt most manufacturers care whether somebody is making a few decals to stick on old bikes they (or their predecessors) made years ago. OEM decals were/are available widely in Europe and even here.
Of course, if you started relabeling Trek bikes "Merckx" and selling them mailorder, you'd probably hear from them rather quickly...
For example, at the Singer shop, you can buy children's bikes and city bikes with Alex Singer decals. Your chance to own a $ 400 (or less, for the kid's bikes) Singer! Are those made by Singer? Of course not. But they figure their customers know the difference. However, I have seen one of the kid's bikes on a Japanese web site with the remark "Alex Singer?"
When buying a vintage bike, you should always worry about the bike being misrepresented, whether it is "upgraded" with "better" tubing stickers, or entirely new decals that purport it being from a different maker, or simply backdated in time with different components. However, most bikes that are valuable enough to make this worth while also are unique enough that you could tell.
Jan Heine, Seattle