Vanish into Memory cares about the creature's power just before it left the battlefield and its toughness just after it returns to the battlefield. For example, if you target a 2/2 creature with two +1/+1 counters on it, you'll draw four cards, then when it returns to the battlefield without any counters on it, you'll discard two cards.

If the creature exiled with Vanish into Memory never returns to the battlefield (because it was a token creature, for example), you don't discard any cards.

If the creature exiled with Vanish into Memory isn't a creature card (for example, a crewed Vehicle), that card will still be returned to the battlefield, but not as a creature. Since the card returned to the battlefield has no toughness, you discard no cards.