Okay, with a bit of research I have have a slightly different answer. I still hold I was correct about the above because I stated that with the information given.
Anyway, with temp HP it is first in first out when it comes to HP gained through the same effect. So, if you cast Vampiric touch one round and then again in another, you use which ever gave you more HP. However, when you cast it as part of the Duskblade class feature to have it effect each creature in the attack, it is counted as one instance of the spell being cast, so all HP damage resulting from that one round of attacks is converted to temp HP.
Doing that again in the next round would only gain extra HP if the Vampiric touch damage was greater than the temp HP remaining from the first round, which would also reset the duration of the temp HP.
Note, you can still gain extra temp HP from other spells and they would stack.
Yeah, WOTC is a mess
The following is from the FAQ
Do temporary hit points from two applications of the same effect stack? What about from different effects? If I have temporary hit points from multiple sources, how should I apply damage?
Temporary hit points from two applications of the same effect don’t stack; instead, the highest number of temporary hit points applies in place of all others. Temporary hit points from different sources stack, but you must keep track of them separately.
For example, imagine a character who gained 15 temporary hit points from an aid spell. After taking 8 points of damage, she has 7 temporary hit points left from the spell. If another aid spell were cast on the same character granting 12 temporary hit points, this total would replace the other spell’s total, meaning the character would now have 12 temporary hit points (rather than 19). If the character then cast false life on herself, she would add the full benefit of that spell to the temporary hit points from the aid spell.
This also applies to temporary hit points gained from energy drain and similar special abilities. Each successful attack counts as one application of the effect (meaning that an attack that bestows 2 or more negative levels still counts as only one application of the effect). For example, a wight gains 5 temporary hit points each time it bestows a negative level with its slam attack. If it bestows another negative level while it has 2 temporary hit points remaining from the first attack, the new temporary hit points would replace the old ones. Temporary hit points are “first-in, first-out.” Damage should be taken off the oldest temporary-hit-point-granting effect first; when that effect is exhausted, apply damage to the next oldest effect. For this reason, you must track each supply of temporary hit points separately.
