I think this is more a suggestion, what about for rarities you have all the stickers for, the button is instead grey leaving the faded button for when you don't have enough stars but this might imply the faded buttons can't be clicked on to reveal the popup "Not enough stars" and I think the other is "you already own all stickers of this rarity"

Intended, it's faded when you cannot purchase the vault because you already obtained all new stickers from them