It's significantly more powerful than other AOE cantrips, but wouldn't be gamebreaking if balanced as part of a specific race
One spell you could compare it to another cantrip that does AOE damage: Thunderclap (Elemental Evil Player's Companion, pg. 168):
Range 5 ft
You create a burst of thunderous sound that can be heard up to 100 feet away. Each creature within range, other than you, must succeed on a Constitution saving throw or take 1d6 thunder damage.
The spell’s damage increases by 1d6 when you reach 5th level (2d6), 11th level (3d6), and 17th level (4d6).
How does your spell compare:
Pro:
- Long range (60 feet, vs focused on yourself)
- Also has the chance to deal 1d4 damage to one target on hit
- Does not have the penalty that the blast can be heard from 100 ft away
Con:
- Less damage (d4 die, vs d6)
Neutral:
- Dex save, vs Con save
- Requires material component
- Piercing damage and thunder damage are both extremely unlikely to be resisted. While many creatures have resistance to "Bludgeoning, Piercing, and Slashing from Nonmagical Attacks", this is a magical attack so it would bypass this resistance.
The cantrips Sword Burst (SCAG p. 143) and Word of Radiance (XGTE 171) are nearly identical. They keep the d6 damage die and the 5 ft range centered on yourself, but switch the save (DEX and CON respectively) and the damage type (force and radiant respectively). This appears to be the standard for a balanced AOE cantrip.
Your spell is significantly better: the reduction in damage from the smaller damage die (which only reduces average damage per target by 1) will generally be outweighed by the chance to damage the primary target twice, while the long range makes this much safer to use (to make good use of damage from Thunderclap, you need to be surrounded by enemies).
It's not hugely overpowered on its own (unless a clever player comes up with synergies that I'm not thinking of now) and if it's a racial feature, it may be alright so long as the race is balanced by being weaker in other ways.
If you want to make it more balanced with other cantrips, I would do some the following:
- lower the range (the 20 ft range for a thrown flask of Holy Water seems like a good figure)
- remove the primary target damage and attack roll (this also has the benefit of reducing the complexity of the cantrip)
Even with those changes, though, it's still going to be more slightly powerful than Thunderclap unless you add an extra downside (maybe the cactus can be caught and thrown back?)
Imbued healing with the healing domain never seems worth the feat. Unlike healing, temporary hit points don't stack, so it'll only ever apply once per target. 1 HP/HD is rarely meaningful.
The following spells are Conjuration (Healing) and trigger augment healing multiple times:
Mass cure X wounds will multiply your augment healing by number of targets.
Darts of life, from Complete Champion, gives you ten darts that heal for 1d8 damage apiece. Augment Healing should let them heal for 1d8 + 10 apiece, and all of the sudden the spell is competitive with heal for raw HP healing, though doesn't remove status effects.
Estanna's stew, from the Book of Exalted Deeds, benefits. It makes one serving per two caster levels, max five, and is 2nd level, so at level 10 you're healing 5d6 + 25 HP for a 2nd level spell.
Healing circle, from Complete Champion, gives you five charges of cure. It's 5th level, so augment healing adds 50 HP of healing here.
Healing spirit (Cleric 4, Paladin 3), from the Player's Handbook II, conjures a little globe that can heal every turn for a total of 1 round per 2 caster levels, though, since it has a duration, extend or persist spell could increase that.
Hoard life, a sorcerer healing spell from Races of Dragon, interacts favorably with augment healing, for whatever sorcerer is mad enough to take the feat.
Magical convalescence, from PHB II, heals you every time someone in the area casts a spell.
Shared healing, from Magic of Eberron, functionally gives an ally lay on hands. Arguably, augment healing would apply every time said ally used the ability. The spell touch of Jorasco, from Races of Eberron, functions similarly.
Warding gems, from the BoED, gives one gem per three caster levels the ability to heal 10 HP. It's 5th level, so augment healing would double the healing. The gems have to cost at least 500 gp, and get destroyed if used, so this is purely for high levels (and it's worse than darts of life until then anyway).
The clear winner to me is darts of life, which is quite strong, and healing spirit, which can be persisted, though the contexts where using persistent spell is acceptable don't really overlap with those where playing a healer is sensible.
Best Answer
I would have the effect of the spell active only on a creature at the time. This way the effect can't be spammed, and the cleric must put real thought into whom to give to.
Since it's two actions I would implement some roll of a dice, e.g. 1d4 + spellcasting ability modifier at second level and after. I don't find this addendum so OP and it gives a nice feeling, and uncertainty, to the cleric that casts the spell, IMHO.
Out of combat, the cleric could use it to someone as they are doing something dangerous, but I wouldn't think much of it. Not all hazards deal only damage - ;) - and even if they do, the damage is still enough to harm the character. At third level, vigor would give (with max ability modifier) in average, 8 temporary hit points, with a max of 12; Fireball Rune, 3rd-level hazard, deals 30 damage on average, with a max of 60. Vigor is useful, as it should be, for the cleric's contentment, but it doesn't break the game, again, IMHO.
In the end, this would be my output:
Vigor
Traditions divine
Cast 2 somatic, verbal
Range 30 feet; Targets 1 creature
Duration 1 minute
You invigorate the target’s body and urge it to press on. You grant the target temporary Hit Points equal to your spellcasting ability modifier. If you cast this spell again on a second creature, the vigor spell on the first creature ends. If you cast this spell again on the same creature, the previous lasting temporary Hit Points are replaced.
Heightened (+1) Increase the temporary Hit Points by 1d4.
Addendum: I thought about comparing the 3rd-level hazard with the 2nd-level Vigor, as spells scale differently. In doubt, I chose the 3rd-level Vigor, since it's the better possibility to consider for the cleric.