I think the spell damage doesn't stack with the weapon damage, while channeling. According to the description for the spell channeling ability of the Duskblade:
...the attack deals damage normally; then the effect of the spell is resolved.
IMO, this clearly separates both sources of damage.
IIRC, the basic rule for Temporary HPs is similar to the one for Circumstance Bonuses, in that they stack if they come from different sources. Following this reasoning, "Vampiric Touch" would still be the same source, regardless of being the same casting. At most, you would get the highest damage dealt by "Vampiric Touch" to the targets you hit, during the full attack, as THPs.
...but i wish i'm wrong!
Please, prove me wrong!
I'm playing a Duskblade as well and this combo would be great!
EDIT: I thought about it for a while, and i can see a way of the Vampiric Touch working, by answering a single question: what is the source for the THPs? Is it the spell, or the creature touched?
If it's the first, it doesn't stack. If it's the latter (sort of implied by the spell's name), it may stack, if the damage is dealt to multiple targets.