Cradle of Vitality

• WWizards of the Coast2008-10-01
  Official Ruling
  You choose a target when the ability triggers. You don’t choose whether to pay until the ability resolves.
• WWizards of the Coast2008-10-01
  Official Ruling
  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.
• WWizards of the Coast2014-02-01
  Official Ruling
  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.

Rulings via Scryfall