For both modes, the value of X is determined as the ability resolves.

If you choose the first mode and there are no other attacking creatures, you’ll create a 0/0 Dinosaur token. Unless something else is immediately raising its toughness, the token will die. You won’t have time to respond with a spell or ability to try and save it. A similar situation occurs if the greatest power among other attacking creatures is 0 or less.

Although the Dinosaur token enters the battlefield attacking, it was never declared as an attacking creature (for the purposes of abilities that trigger whenever a creature attacks, for example).

You choose which player, planeswalker, or battle the Dinosaur token is attacking. It doesn’t have to be the same player, planeswalker, or battle that Ghalta and Mavren is attacking.

If you choose the second mode and there are no other attacking creatures, you won’t create any Vampire tokens.