I read that the numerical value of John Paul II in Latin is 666. How come World's Last Chance is applying 666 to Vicarious Filii Dei instead?

This is a good question. Those who have arrived at 666 for the Latin name of John Paul II are using an incorrect name: Ioannes Paulus Secundo. This is not the correct Latin name for the pope. It is true that the numerical value for it is 666, but since it is not the correct Latin name we can not use it. Secundo is in the dative/ablative singular and not in the nominative singular that it should be when used after Ioannes Paulus. The correct Latin name for John Paul 2 is: Ioannes Paulus Secundus , John Paul the Second, and the numerical value for this Latin name is: 671.