I think that, so far as UKn's are concerned, "contrail" is a fairly technical term. I first heard it as a member of the Royal Observer Corps, back in the late 50s, and my wife was familiar with the term from her work for BOAC (the direct descendant of Imperial Airways!) so it was well understood in our household. However, I have had to explain the word to other people from time to time, whereas "vapour trails" is reasonably self-explanatory.