If there are no creature cards in your graveyard, the target creature just gains vigilance until end of turn.

Because tokens aren't cards, they never count for undergrowth abilities.

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.

If Vigorspore Wurm gains menace, it can't be blocked at all.

Creature cards with other types, such as artifact creature cards, count for undergrowth abilities.