Whenever you gain life, you may pay. If you do, put a +1/+1 counter on target creature for each 1 life you gained.
10/1/2008 You choose a target when the ability triggers. You don’t choose whether to pay until the ability resolves.
10/1/2008 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.
2/1/2014 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.