Things that improve healing (magicka-based):
- Spell power
- Spell critical
- Max magicka
- CP: Blessed (healing)
- CP: Elfborn (spell crit damage/healing)
- Mundus: The Apprentice (spell power)
- Mundus: The Mage (max magicka)
- Mundus: The Ritual (healing)
- Mundus: The Thief (crit)
Special cases:
- CP: Bastion (shields) - strengtgens shields, so also increases the heal of Healing Ward provided the shield is not destroyed or overridden, since that depends on the size of the remaining shield
- Weapon damage, weapon crit, max stamina, and associated mundus stones - stamina based heals like Vigor use these instead of the magicka and spell valies
Of the options, I recommend focusing on increasing spell power and/or max magicka, as these are more useful universally for all magicka spells.
A healer should probably focus on around 50% spell critical (give or take). More is overkill unless you are off healing on a magicka dps crit build.
The only fight I ever even consider slotting Vigor for is the second to last boss in veteran Fungal Grotto, since it drains health and magicka. With most groups my healer's plan of timing potions/ultimates is sufficient, but if a dps has health way below what they should I will consider slotting it 'to be safe' (and typically only if we wipe the first time). As soon as the fight is over, if it was on my bar, it's off again. It doesn't have a place on a healer's bar.