I have run into another problem with Vilja, as she won't use the bow I gave her at all. I had aquired the Frostwyrm Bow from the Udefryte Matron, and gave it to her. I also supplied her with several dozen elven arrows. Even though I tell her to hang back and use her bow, she frequently draws her sword and charges the enemy like a berserker. I am currently in Fort Rayles, and there is one section where there are three of those arrow traps in one corner (along with several Guardians of Oblivion OOO adds). Is this is an example of the bow issue you mentioned in your first post, Emma?
Next time I load that game, I am going to have Vilja wait until that corridor is safe.
No... the bow issue I'm refering to is that all companions I have seen, and also regular bowman npcs, occasionally stop using arrows in their bows - they fire without arrows and doesn't make any damage. You might have to switch cells to make this work properly again.
Is she using her bow at all, or only a sword? If she uses her bow a little, and then too early switches to sword, I wonder what the area looks like...
What I have seen with corridore systems where heigh level varies is that companions tend to calculate the distance to the enemy following a straight line x->y (sorry, my english isn't sufficient...

I'm sure there's a expression for it). So an enemy that for instance is very much below you but close to you seen from above will by Vilja be considered as 'very close', i.e. she will attack him with melee-weapon even if she is set to bowman, as she thinks he is very close.
You could try and set her aggression to 0. (Let's avoid fighting). Then she will only fight if a) the enemies are damaging her health AND B) you have drawn your weapon.