Bev, I'm fascinated by your account of the tu/vous dilemma with respect to your parents-in-law. I have never heard, or heard of, a natural child of either gender, addressing one of his natural parents as 'vous', irrespective of age. I'm trying to recall if there isn't a class and/or period factor (from previous reading) that can affect this, but it seems unusual to me. Aside from royalty and the haute bourgeoisie, of course!
Distance (not physical, but in relationship terms) can affect how one addresses parents-in-law. It is more natural to start off with 'vous'. Sometimes closeness can bring you to feel that tu is more appropriate, and by way of example, I began to 'tutoyer' (and he me) my father-in-law after my first marriage very early on (I was in my early 20s, he was a laid-back, jazz piano-playing, opera singing, debonair guy who acted as if he was a teenager right through until his 80s). In a way, he would have felt awkward had I used 'vous' - it would have aged him. His daughter was in a serious road accident before we reached our first wedding anniversary, but 'tu' remained the preferred form of address whenever we spoke, until he died.
Second time around was different. Already in my 30s, 'vous' felt more comfortable; although my father-in-law and I instinctively knew we would have been ok with 'tu', his stand-offish wife wouldn't have liked it. Then following their daughter's unexpected death, after nearly 20 years, he and I suddenly began to 'tutoyer' each other; it was an empathy thing. MIL and I grew if anything even further apart, so 'vous' it remained; no chance of that ever changing.
Vous seems to me to be perfectly natural and acceptable in most parents-in-law situations, I can't imagine how it would be seen to be otherwise - it's a natural term of respect between parents-in-law and gendre/bru, until/unless parents-in-law suggest otherwise (it's more their place to do so, an age and respect thing).
All to describe how diffficult it is suggest some kind of definitive rule governing the 'tu'/'vous' issue, except in more straightforward cases (such as with vous in a child addressing an unrelated adult, or in subordinate business relationships, or when first being introduced to a stranger, etc).
Class and region are factors. Chatty paysan southerners are more likely to use tu than vous, early on, especially with their fellows. But it's more awkward with outsiders/foreigners where vous feels safer much of the time, at least before people get to know each other properly. Mountain folk, the montagnards, are harder to get close to, so vous is used more frequently. In cosmopolitan areas, the edges to these 'rules' become blurred.
Generation is another factor - peer groups of university students would sound very odd vousvoying each other, even shortly after they met for the first time. Then again, when the class factor is factored in, to add a little nose-in-the-air distance, amongst students of a certain background vous may be the standard form employed.
As for the use of tu/vous to indirectly express approval or disapproval, how very true. In a situation when you would normally use vous, suddenly switching to tu can be very insulting, depending on the tone of the conversation. Or if someone tries to be prematurely familiar with a 'tu', a deliberate 'vous' in reply can be quite a putdown.
I've sometimes heard foreigners using tu with abandon and clearly inappropriately, and after years of immersion it feels unnatural and even rude to me at times. Not so much when it's obvious that the foreigner has just arrived and barely speaks the language, but when he/she has been around for a while and can express him/herself clearly. It sounds insensitive, boorish, as if the foreigner hasn't taken the trouble to understand how it works.
So to those unfamiliar with this highly complicated situation, a rule of thumb is to always use 'vous', except when addressing children up to say at least mid teens, and until/unless invited otherwise.
