Slightly different answer 2 years later since I was looking for this, and I noticed a slight error in the previous answer concerning the wording.
Specifically: "...and that spell directly heals another creature when you cast it..."
When you cast a spell ... that restores hit points to a creature other than you...
- The Blessed Healer ability doesn't limit when the spell has to heal other creature, just that it does, and so, as long as you heal another creature with the spell, you should get 2+spell_LVL HP the moment you cast it, even if the healing effect of said spell happens in the future.
- So as long as you heal someone with it in the future, you should get the heal from BH to yourself at the time of the casting. Wonky, right?
- I repeat, the wording doesn't specify that the healing spell you cast has to heal on the same turn as you cast it, so the moment you would've healed someone with Aura of Vitality for the first time, you should've gotten healed for that 2+spell_LVL HP backwards in time when you casted the AoV spell since you fulfilled the BH condition.
- Either way it would still only give you 2+spell_LVL once since you cast the spell once, not every time you use the bonus action to heal.
I myself would rule that you would get the Blessed Healer healing the first time you use the aura on a creature other than yourself.
If this is confusing or unconvincing, I'll break the language of Blessed Healer into parts:
- When you cast a spell - TRIGGER
- (spell) of 1st level or higher - CONDITION 1
- (spell) that restores hit points to a creature other than you - CONDITION 2
- you regain hit points - EFFECT
- equal to 2 + the spell's level (effect specifics)
Entirelly stripped bare sentence is therefore:
"When you cast a spell, you regain hit points."
and that is what in Blessed Healer is time restricted by "when you cast a spell."
The CONDITION 2 has no time restrictions in terms of suddenness, just that it "...restores hit points to a creature other than you...". Where restores word only sets that it has to happen during OR after the casting. (or even technically the past if such a thing would be possible, a sceneario I can imagine out of my head is you somehow watching past events and being able to interact with them while still being in the present, like looking through a portal or something, and you heal someone there, altering history)
To pull out a non-D&D english sentence with similar wording
- "When you sell a chocolate with 4 bars that makes someone happy, you get money for it."
Just like with the wording of Blessed Healer, the TRIGGER and EFFECT would be "When you sell a chocolate, you get money for it."
while the CONDITION1 would be "(chocolate) with 4 bars"
and the CONDITION2 would be "(chocolate) that makes someone happy" and just like "(spell) that restores hit points to a creature other than you" it is not restricted by when does it even make someone happy, just that it does.