I would refer you to Fred's previous comments about damage over time effects. In summary he's said they're boring and rarely important to the narrative. The more I think about it the more I like the maneuver. It's easier, requires less fudge work and really works well to create a great story.
As an example let's work out this one situation both ways. With damage over time effects it works like this: You deal the vampires x stress. Next exchange you deal the vampires x stress. Next exchange you deal the vampires x stress. It's not very great from a story standpoint.
On the other hand the maneuver creates a situation that the GM can create something great with. "The lesser vampires ignite, screaming as they try to dive for cover (taken out lethally) . One of them makes it under a car, his blackened and twisted shape shaking and moaning (taken out non-lethally). The stronger ones become a blur as they find the edges of the brilliant light and hide, smoking in the shadows, waiting for an opportunity to strike (compelled to leave the zone, and/or possibly to deal with an attack of some sort). Some run from the bright light leaving the PCs alone (compelled to concede, accepting consequences, or simply failing at whatever objective they had).
I (and Fred for that matter) simply prefer the story focused method.