Midrange decks have lots of removal, interaction, and strong creatures
The one cost instant will give River Song a +1/+1 to grow permanently and then phase out for a turn as response to removal targeted like Swords to Plowshares, mass like Blasphemous Act, or combat damage
It is the same on all cards, whether they are Effect Monsters, Spell Cards, or Trap Cards
Its a happy table that plays games that are about more than just casting the most powerful spells in your hand as fast as you can
I could see this going in a deck with Elenda, the Dusk Rose or Teysa, Orzhov Scion
Xyz Encore could still be activated even while Vanitys Fiend effect is applied, Xyz Materials will be detached and the Xyz Monster will be returned to the Extra Deck, but the monsters among the detached Xyz Materials will not be Special Summoned