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 Vigean Hydropon with two +1/+1 counters on it, you’ll draw two cards, then when the Hydropon returns to the battlefield with five +1/+1 counters on it, you’ll discard five 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, it was an animated Gruul War Plow), 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.