Vadrok, Apex of Thunder

• WOTC Official Reminder Textofficial rules
  Mutate — If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.

• WWizards of the Coast2020-04-17
  Official Ruling
  If you cast a spell “without paying its mana cost,” you can’t choose to cast it for any alternative costs. You can, however, pay additional costs, such as kicker costs. If the card has any mandatory additional costs, such as that of Swallow Whole, those must be paid to cast the card.
• WWizards of the Coast2020-04-17
  Official Ruling
  If you wish to cast the target card, you do so as part of the resolution of Vadrok’s ability. You can’t wait to cast it later. Timing permissions based on a card’s type are ignored.
• WWizards of the Coast2020-04-17
  Official Ruling
  If a spell has in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

Rulings via Scryfall