The prop that becomes the VIP, whatever prop they're disguised will have their material overwritten with a golden material [console: material_override models/player/shared/gold_player] which fellow props would see
but hunters wouldn't (seeing the original material as usual).
Obviously limiting the material change to be seen by 'Props(/Spectators)' only as would be unfair if hunters could always easily spot out the VIP (unless VIPs are given a sort of boost but that's just an afterthought) or possibly hunters can only see it while dead. The main issues that would arise from this could be the difficulty in coding, though I've been told it's could still be possible to do, and material issues if they don't have TF2 content (though this could be resolved by certain addon(s)).
What 'Hunters' (possibly dead excluded) would see:
prntscr.com/kmiwef
What 'Props' (both alive and spectating) would see:
prntscr.com/kmiufg
Didn't quite get the time to finish off the concept images so had to use these.