The VIP sticker may be enough to tell you and the owner of the tank that it may not contain air, but is it sufficient labeling to prevent the inadvertant use of the tank by a diver that has OW certification and nothing else?
The intent of the bumper sticker is to be obvious enough that there is negligible chance of it being erroneously used by another diver that thinks it is air. When you think of the bumper stickers as performing this function, then it makes sense that many fill operations require bumper stickers on single tanks, but don't require them on a double set.