It can start spawning on items at Item Level 47, but will not be able to spawn on all items with full effect until level 91. Before this item level, +40 to Life is the highest possible roll.
Vita is the most sought after suffix for Grand Charms that have a +1 to class skills prefix (such as Lion Branded). Due to the very steep requirement of item level 91 needed for a Grand Charm to spawn +41 or higher to Life, only two monsters are able to drop such Grand Charm - Hell Nihlathak and Hell Baal. This makes "41+ Life skiller" Grand charms extremely rare and very expensive.
The common (yet still expensive) strategy of obtaining such Charm is getting a Grand Charm drop from Hell Baal, then using a Horadric Cube recipe of 3 Perfect Gems + Magic Item for an affix reroll on the Charm, until desired affixes are rolled.
+16-20 To Life (Small)
+26-35 To Life (Large)
+36-45 To Life (Grand)