How does Sneak Attack damage apply to Vampiric Touch?

Question

Inspired by the question Does sneak attack class feature apply for ranged touch attacks? asked by ayvango. Under that question, it is made clear that in D&D 3.5 Sneak Attack damage indeed applies to spells that require an attack roll.

This is a newfound boon for my Daggerspell Mage character I have not been aware of.

Yet there are some things that answers to the previous question did not clear up for me. One such is: how does Sneak Attack damage apply to Vampiric Touch? Is it above the damage cap of max 10d6, and does it convert to temporary hitpoints?

My interpretation is that Sneak Attack damage is above the cap and does not convert to temporary damage (as it is not the spell's damage).

Is this correct? If not, why? (Any official reference is welcome.)

Answer 1

Is it above the damage cap of max 10d6?

According to the entry for vampiric touch:

Your touch deals 1d6 points of damage per two caster levels (maximum 10d6).

The cap is just a maximum that can be added by caster level.

Does it convert to temporary hitpoints?

You gain temporary hit points equal to the damage you deal.

The sneak attack damage does qualify for temporary hitpoints.

However, you can’t gain more than the subject’s current hit points +10

But you are still limited to the same maximum temporary hitpoints

Answer 2

TL; DR

Sneak attack damage (also critical hit damage) can go above the maximum, and you get the temporary HP from sneak attack damage (and critical hit damage).

Spell Description

Your touch deals 1d6 points of damage per two caster levels (maximum 10d6). You gain temporary hit points equal to the damage you deal.

Explanation

The cap is referring to the damage increase from caster levels, meaning that it stops increasing in power at caster level 20. This is the base damage. Any damage boosting coming from a source other than caster level would not be affected, including sneak attack.

Justification

Most spells increase in power with caster level. They may increase in range, be harder to resist, deal more damage, etc. For damage dealing spells, this increase is primarily through either additional dice (like with Vampiric Touch) or additional "hits" like with Acid Arrow. Either way, the increase is capped at some point. In the case of Vampiric Touch, it's capped at 10d6. Acid Arrow is capped at 6 extra "hits" (rounds of damage).

So if sneak attacks (and by extension critical hits since they both add extra damage) were capped by the listed maximum, at high levels they would be at a significant disadvantage to spells that were capped in a different way. A level 20 casting Vampiric Touch would in no way benefit from a critical or sneak attack. However, Acid arrow would still be eligible to receive the full benefit of a critical or sneak attack.

Temporary Hitpoints

Since the temporary hitpoints gained are based on the damage you deal, you would gain the full amount (within the normal limit of max HP +10), including the extra damage caused by sneak attack.