From what I've seen, Shadow Fire spawns close by to the lit-up patches, whether inside or outside a bit seems as if it happens equally, with the positioning that Demonicsothe explained. Just steer way clear of any orange patches and you'll hopefully avoid them.
You mentioned that some runs seem to have little fire at all. I'm assuming this is because the player that Vanaduke is aggro'ed on is staying out of his view, possibly attacking him with a Blitz if someone is Shiver'ing Vanaduke. Vana attacks when he sees that aggro'ed player, creating more shadow fire. You can take advantage of this by grabbing Vana's aggro and constantly staying behind him when attacking, so you have time to kite him when he turns around.
I've also noticed that when my party is watering Vana's fireballs, there usually aren't many fire patches around. However, a few water balls can usually clear out patches of fire, so I'm guessing that maybe the fire was already cleared beforehand. Usually, when I notice a build up of Shadow fire, players tend to be too busy watering it, either attacking Vana, slags or lying down.
With your suggestion, I'd find it more convenient; especially without shadow fire spawning on knights, but some players would argue that Vana is easy enough already. Shadow fire isn't really much of an issue if the tide of the battle is in your favour, but can be pretty annoying if the opposite.

Another observation I made is that the maximum number of shadow fire tiles may be related to number of players. I did one run with my alt and another knight, and there seemed to be a lot more fire.
Ah. That would explain the endless hell of fiery doom when vanafights go pear-shaped.
Oh, with the standing and smacking part, what if vana does a trojan boom and sends streams of fire across, or tries to rugby tackle someone?