The amount of life you pay is chosen when Vizkopa Confessor's last ability resolves.

If the target opponent is an illegal target when Vizkopa Confessor's last ability tries to resolve, it won't resolve and none of its effects will happen. You won't pay life and no player will reveal any cards.

You may pay 0 life when Vizkopa Confessor's "enters" ability resolves. If you do, no cards are revealed or exiled.

The amount of life you gain from extort is based on the total amount of life lost, not necessarily the number of opponents you have. For example, if your opponent's life total can't change (perhaps because that player controls Platinum Emperion), you won't gain any life.

The extort ability resolves before the spell that caused it to trigger. The ability resolves even if that spell is countered.

You may pay a maximum of one time for each extort triggered ability. You decide whether to pay when the ability resolves.

The extort ability doesn't target any player.