The range of the Shivermist is large enough to reach beyond his fire.
I've been running FSC with guildies for a while now, and never once have I nor any of my teammates had to water Vanaduke in order to freeze him.
Lately, however, I've run into an increasing number of knights who don't seem to realize this. Just today an FSC party I was in went down the gutter in part because the designated bomber was too busy watering Vana to actually bomb. (I don't have experience killing Vanaduke without shiver, that's a topic for another discussion.)
Point is: Don't waste time watering Vanaduke. Seriously.
EDIT: Okay, so not totally pointless. I stand corrected. It's just not totally necessary either.
Actually, having your slagger water vanaduke after watering the floor is the right thing to do. Afterwards you can have your slagger aim a well-timed flourish charge into Vana, dealing 1.4 if not 1.6k damage