Vampiric Touch spell states "you gain temporary hit points equal to the damage you do". Hellcat Gauntlets deal 1-6 slashing damage per spall level on a single target spell. Would you gain the hit points from both spell and magic item damage?
Your DM would have to make a ruling.
If he agrees that the slashing damage becomes part of the spell damage, read below:
- Hellcat gauntlets is effectively adding damage in addition to the spell, as part of the spell.
If he decides that the slashing damage is separate from the spell, read below:
- Hellcat gauntlets is effectively adding damage in addition to the spell, not as part of the spell.
The text of Hellcat Gauntlets can be deceiving and therefore would require an authority figure, such as your DM, to make a decision and remain consistent with that decision; "Hence forth... Hellcat Gauntlets' slashing damage is [insert decision]."
Hellcat Gauntlets: When you activate hellcat gauntlets, the next spell you cast during your turn that targets a single creature also deals 1d6 points of slashing damage per level of the spell, unless the spell has no effect on the target (due to spell resistance or a save negating the spell, for example), in which case the damage is negated. [emphasis: bold]
Note: If he decides the extra damage becomes a part of the spell's damage, this would be great when used with the Sudden Maximize1 feat, or similar effect.
1Once per day, you can apply the effect of the Maximize Spell feat to any spell you cast without increasing the level of the spell or specially preparing it ahead of time. You can still use Maximize Spell normally if you have it.
The description states that the gauntlets cause the spell itself to deal extra damage; if the extra damage was meant to be separate from the damage produced by the spell itself, the description would state that when activated 'the gauntlets' deal extra damage to the target affected by the touch spell equal to the level of the spell cast (instead of specifically stating that the gauntlets cause the spell itself to deal extra slashing damage).