You choose a target when the ability triggers. You don’t choose whether to pay until the ability resolves.

Cradle of Vitality triggers just once for each life-gaining event, whether it’s 1 life from Deathgreeter or 8 life from Soul’s Grace. You pay the cost just once, but the targeted creature will get a number of counters equal to the amount of life you gained.

If multiple creatures you control with lifelink deal damage simultaneously, each instance is a single life-gain event and the ability will trigger separately for each of them.