I just wanted to state the fact that considering the explanation of a french word people should listen to the nativ French not the Swiss-German (me).

And in relation to the auld alliance I would see two reasons why to use vieille instead of ancien:
1. It is an alliance that still exists in the mind of many scot. (Even Billy Connolly mentioned it on his tour around Scotland)
2. ancien alliance is at least to my knowledge bound to the old testament use. A reason to call the auld alliance vieille alliance even if it might be a strange use of word.