I'll have to check on the dps of vt vs mf, but the idea was to have dots tick away as you're casting other spells. VT will always be useful with FDCL and multidotting. They said before they want multidotting to be nerfed, so our dots are near garbage. But maybe if your comparison is true, maybe they'll finally buff them.
I don't think they want to give us a dps cd because they have said they wanted to clear up the clutter, and we have enough complexity as it is, unless they tack on an additional boost to VE or such.
I think blizzard knows they have to boost our single target damage. I doubt it will be MB or MS since talents help them out. It will have to be MF (20%), which will certainly make your argument stronger, hence needing to buff our dots, conflicting with making multidotting stronger. It usually takes Blizzard 3 months to figure out the class balancing.
This should fix it.
Vampiric Touch - Does X amount of shadowdamage over 15 seconds and regenorates 2% of your mana every time it does damage. If affecting more then one target the total damage of your touch will be reduced to 90% on each additional target.
(add with pain and problem fixed)