Is the homebrew weapon attack cantrip ‘Vampiric Assault’ balanced?

I have created some new cantrips which include making a weapon attack as there are only two official ones to choose from. I’ve tried to follow these principles based on Booming Blade and Green-Flame Blade:

  • Melee weapon attack only, 5 foot range
  • No unavoidable extra damage to the primary target at level 1
  • Damage no more than 2 targets
  • Spellcasting ability modifier doesn’t have to affect the spell at all
  • Only available on the Warlock, Sorcerer and Wizard spell lists

But with such a small amount of official content to compare them against, it’s hard to say if they’re balanced.

Is this spell balanced with Booming Blade and Green-Flame Blade?

Vampiric Assault

Necromancy cantrip

Casting Time: 1 action
Range: 5 feet
Components: V, M (a weapon)
Duration: 1 round

As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature within the spell’s range, otherwise the spell fails.

On a hit, the target suffers the attack’s normal effects, and if it takes any damage from the attack, you gain temporary hit points equal to your spellcasting ability modifier (minimum 1). The temporary hit points are lost at the end of your next turn.

At 5th level, the attack deals an extra 1d6 necrotic damage to the target. The damage roll increases by 1d6 at 11th level and 17th level.

If someone casts Vampiric Touch on a creature with necrotic absorption, does the caster take damage?

Leaving aside the question of if there’s any way in the manual for anyone to have necrotic absorption in 5e (which as far as I know isn’t the case), and given the text of Vampiric Touch as follows, would attacking a creature that heals from Vampiric Touch do damage to the caster? Vampiric Touch:

Make a melee spell attack against a creature within your reach. On a hit, the target takes 3d6 necrotic damage, and you regain hit points equal to half the amount of necrotic damage dealt. Until the spell ends, you can make the attack again on each of your turns as an action.

It has been brought to my attention that damage absorption is not actually a keyword. The following explanation is sourced from The Rise of Tiamat (aka Tyranny of Dragons) campaign, attached to the dragon masks.

Damage Absorption. […]whenever you are subjected to damage of that type, you take none of that damage and regain a number of hit points equal to half the damage dealt of that type.