I couldn't find anything in the FAQs, so I'm going with the wording of the rulebooks on this one:
Fire Storm
Fire Attribute
If a direct damage or magic missile spell from the Lore of Fire is cast at a unit that has already been hit by a direct damage or magic missile spell from the Lore of Fire in the same Magic phase, the casting Wizard adds +D3 to his casting total
Fire Storm will not gain the +D3 to cast, because it does not specify a target, it simply drops the template. Even placing the template directly on top of an enemy regiment does not count as
targeting that regiment. However, any regiment that is
hit by the template will be eligible for the +D3 from another Fire spell such as Fireball.
Soul Stealer is much the same, because it specifies that only units
targeted by the spell gain Vengeance Counters, but that any unit
hit by a spell can expend Vengeance Counters. Dark Magic is lucky therefore, because you can throw small spells first, and then gain the effect with the larger spell (Soul Stealer). Targeting a regiment with DoomBolt, for example, will give them a Vengeance Counter. If you then place the Soul Stealer template in such a way that it scatters onto that regiment (hits it), you may expend Vengeance Counters to have Soul Stealer deal out additional damage. However, just like Flame Storm, Soul Stealer does
not designate a target, and will not add Vengeance Counters to any regiments which it affects.