It has been over a year, so I think you did a good job. Here's my more complete (and slightly revised) explanation of her three combat modes:
Aggressive: She'll attack on sight, like just about every other companion and NPC. The enemies won't target her as much as they do other companions though.
Passive: If you tell her to avoid combat, she'll ignore combat unless the player is very wounded (down to about 30% I think) or some enemy actually hits her directly. She'll usually stand a little farther away during these situations so she doesn't get in the way as much. Enemies won't usually target her, so any hits are usually from area attack spells or some accident. (This isn't quite as true for OOO or FCOM users, but that's an issue with the way OOO changed the game factions.)
Defensive: I consider her default strategy the most useful because she will attack but tends to stay out of the way. She will only attack if an enemy hits her or the player gets wounded (not on the first hit though) or if the player has a weapon drawn. If you play a mage character you may want to think of raising your fists as a signal that you want her to attack. And make sure you put your weapon away between fights, or she'll stop paying attention to it.
When you draw your weapon (or just raise your fists), I bump Vilja's aggression just slightly to give you a way to signal that combat is starting and she should join in. But because some people have the bad habit (myself included occasionally) of entering a village or city with a weapon still drawn, I also lower her aggression again after the weapon has been out for a while. I think it makes perfect sense. If I saw you draw a weapon it would make me more alert for danger, but if you continued to walk around with a sword drawn I would eventually get tired of being on alert. If you constantly run around that way, you obviously just like the look and feel of the shiny sword in your hand.