Vega is probably the most important hero to have if you are using an army that can be healed (any unit that is not a mechanical or undead unit) Vega and her ability favor of the sun god heals all allied forces on the battlefield which is frankly overpowered.

Heal (costs 1 mana)
Vega casts healing on the specified area, healing friendly troops and causing them to recover X HP. Has no effect on Undead, Mech, and Building Units. Bonus based on Magic
Get one point in this ability just to spot heal your army and apply the boosted healing ability (which reduces damage taken by 30% for 4 seconds)

Mana Barrier (costs 2 mana)
Vega summons 1 magic shield with X HP that can effectively block the enemy’s ranged archers and ranged magic damage, lasts for X seconds. Bonus based on Magic
This ability absorbs incoming ranged attacks such as arrows from archers, it really helps save your army.

Saving Lives
Vega boosts the troop injury rate by X%, saving more troops from death in battle. (8% at 8/7)
Maxing this skill and having Elena’s saving lives at 8/7 as well and having +10 injury reduction as a bonus perk from King’s Court allows you to have 96% recovery chance while in enemy lands!

Priestess Mastery
With the blessing of the Sun God, when Vega is on the battlefield, all allied Priestesses, Abbesses, and Saints have damage and max HP increased by X%. Bonus based on Command (32% at 8/7)
Keeps your priests alive longer, your priests don’t attack often as they are busy healing so this skill isn’t really needed you are better off getting saving lives.

Boosted Healing
Allied units that are healed by Vega’s abilities will also get a shield that allows them to reduce damage taken by 30%, as well as having a healing effect for X seconds. Bonus based on Magic
You only need one point in this skill each point more only adds 1 second to the buff which isn’t worth the ability points.

Favor of the Sun God (costs 5 mana)
Vega summons a Sun God Blessing for allied troops, causing all allies to continuously recover HP for X seconds. Has no effect on Undead, Mech, and Building Units. Bonus based on Magic
THIS SKILL IS LITERALLY GOD TIER IT IS SO IMPORTANT TO ANY ARMY THAT HAS NONE UNDEAD/MECHINICAL UNITS THAT I AM PUTTING THIS COMMENT IN CAPITALS.

Building Vega
Vega is typical healer, so she is very squishy but provides a large range of support spells. She needs only magic as she will die if anything touches her, so just maximize her magic stat and maybe some stamina so you can use her one or two more times a day.
You want to get one point into Vega’s base skill “Heal” and use it only when you need to spot heal a group of units to keep them from dying, this spell is used more to provide “Boosted Heal” rather than the healing itself. This means get one point in Boosted Heal. After those two skills put all your points into Favor of the Sun God, make sure this skill is as high level as you can get it at all times, after that dump everything spare into Mana Barrier. If you are playing super late game put the rest into saving lives.

Equipping Vega
Refer to Avril for equipment for early/mid game, just stack as much magic as possible. Late game you will want to equip all of Vegas plus to ability items as she has quite a few of them and they are good to have, ESPECIALLY THE ONE THAT GIVE +1 TO FAVOR OF THE SUN GOD.