This song has a female voice pronouncing only the syllable "la", and there are strings (violins, cellos, etcetera). It is not very old, perhaps from the 1970's, and probably from France. I remember reading (many years ago when I knew it's name) that it was described as "contemporary classic music" or "modern classic music".
There's a version of the song Yeul's Theme from Final Fantasy XIII-2 where all the lyrics of the song are replaced for La. It is a female vocals with strings (and also a Piano). It matches most of your specifications.
As Angst did back in September, I immediately thought of the Villa-Lobos piece. But I wonder if it might be "Un homme et une femme" by Francis Lai.
It's from the 1966 film of the same name and it's isn't any kind of classical music, but people often describe music wrongly. It's French chanson really, with a Brazilian, bossa-nova feel. I'm afraid this arrangement - which is probably the original - isn't all strings, but if it's the right song you could search for other arrangements.