In the example of the wizard blasting the oncoming vampires, I think targeting this as a zone would be appropriate. After all, it is not a combat yet. Plus, I would probably put multiple zones down, if the room was large enough. A zone doesn't need to be divided by walls or fences, and it would make sense, that you would have to run through the room, in order to affect anything there, so you change the zone. In fact, any time there would be a reason for at least a +1 movement action to get to the action, it would be a different zone.
Maybe the size of a zone is affected by the kind of the conflict. In fact, when the vampires arrive, it might be a social conflict, before the wizard throws down, so a "us" zone and a "them" zone might be perfectly acceptable, though blurrier than other zones. Once it turns into a physical conflict, you might not be able to distinguish like this and it turns into the usual zones.
In a conflict, the parties don't stand still, and even if the vampire is standing 10 feet away from your team mate, by the time the fiery beam hits, he might already be over and hitting him. That is why zone attacks hit everyone in the zone. Dishing out fire is easy, controlling where it goes is hard. Maybe you could take another -2 shifts to shield someone from the effect, if he is inside the zone. That way, anyone could do a "duck" maneuver to get out of the way, that they can let you use to negate this effect. If they don't, it is going to be harder to not hit them. I hope you get what I am trying to say, I have the feeling I don't make much sense right now.