Then there is the combination of the above examples, which would be something like making the opposed maneuver Silverblaze mentioned to cover everyone in the warehouse in holy water (a generally non-harmful thing to do, unless you shoot it like a pressure cannon or try to drown them in it, so it shouldn't hurt the captives) and then saying that any vampire hit takes X stress (maybe the ~weapon 3 Sanct mentioned, though if the power mentions some sort of rule for how this works use that instead). You could even say that it is a maneuver against people to apply the Soaked aspect and a zone attack against vampires because you are making a harmless action against mortals but a harmful one to vampires.
All these methods have different strengths and weaknesses. Silverblaze's may be the most powerful, since every subsequent attack from everyone now ignores all their toughness and recovery (in one round of combat, this is likely to be at least 5+ stress and making it easier to inflict consequences), but it doesn't do damage up front. A strait attack may or may not be worse than just 3ish stress.
It is really up to the GM to determine how powerful he wants this type of effect to be. If you are worried that the wizard will overshadow the other characters by doing this, or will make the fight you spent an hour setting up into a cake walk (and thus boring) opt for a less powerful option. If everyone at the table thinks this maneuver is soooo cool (especially if all the other PC's helped the wizard set it up) then let it be strong. Just don't let the wizard solve every conflict in 1 round with this sort of effect, it is the type of thing that only works every so often (maybe next time the RCV's wore raincoats
)