Undergrowth — When Vigorspore Wurm enters the battlefield, target creature gains vigilance and gets +X/+X until end of turn, where X is the number of creature cards in your graveyard.
Vigorspore Wurm can't be blocked by more than one creature.
Because tokens aren’t cards, they never count for undergrowth abilities.
Creature cards with other types, such as artifact creature cards, count for undergrowth abilities.
If Vigorspore Wurm gains menace, it can’t be blocked at all.
If there are no creature cards in your graveyard, the target creature just gains vigilance until end of turn.
The value of X is determined only as the undergrowth ability resolves. If the number of creature cards in your graveyard changes later in the turn, the target creature is unaffected.