The choice of creature type is made as Vanquisher's Banner enters the battlefield. Players can't respond to this choice. The bonus starts applying immediately.

The last ability of Vanquisher's Banner resolves before the spell that caused it to trigger. The ability will resolve even if the creature spell is countered.

To choose a creature type, you must choose an existing creature type, such as Vampire or Knight. You can't choose multiple creature types, such as "Vampire Knight." Card types such as artifact can't be chosen, nor can subtypes that aren't creature types, such as Jace, Vehicle, or Treasure.