You still choose which player or planeswalker Valley Dasher attacks.
If, during your declare attackers step, Valley Dasher is tapped or is affected by a spell or ability that says it can't attack, then it doesn't attack. If there's a cost associated with having Valley Dasher attack, you're not forced to pay that cost, so it doesn't have to attack in that case either.
If Valley Dasher enters the battlefield before the combat phase, it will attack that turn if able. If it enters the battlefield after combat, it won't attack that turn and will usually be available to block on the following turn.
