I realize it isn't French, or Campagnolo, but I recently changed out the Mighty Comp crankset for a Mighty Tour on my '72 Finest to get 34/48 gearing up front. The SunTour Vx that's on the rear, and the Compe-V on the front handle everything quite well. The Compe-V is a reverse action front derailleurs, and I find it does a remarkable job of shifting under moderate load.
Faster? Possibly. Better? Likely. Cheaper? Definitely. :)
I don't think I'd slap these on a constructeur, or a steed with an overwhelming national heritage, but since you didn't mention the lineage of the intended...
Mechanicsburg, PA, USA
> From: Charlotte Bronte <email@example.com>
\r?\n> Subject: [CR] Best vintage derailleurs for a 50/34 compact setup--French or Campy
\r?\n> To: firstname.lastname@example.org
\r?\n> Date: Wednesday, May 6, 2009, 10:07 PM
\r?\n> Taking the plunge into compact gearing for my vintage
\r?\n> touring project. No less an authority than the late
\r?\n> Sheldon Brown (see his Hetchins) recommends it, but he uses
\r?\n> a modern long cage rear derailleur, which I'd like to
\r?\n> avoid if possible.
\r?\n> I can go either way, Campy or Simplex/Huret. I'll be
\r?\n> running a TA or Stronglight 50/34 on the front, and would
\r?\n> like to know from members who use such a setup about best
\r?\n> derailleurs. Have followed the posts here about Rallys with
\r?\n> interest. That's one way I'm might go, either with
\r?\n> a pure Rally or a NR fitted with a Rally cage. I've got
\r?\n> many replies about this.
\r?\n> Another option is French, either Simplex or Huret, but here
\r?\n> I'm completely out of my depth. What is the maximum cog
\r?\n> you can run with these derailleurs, and should I be looking
\r?\n> for a long cage version of the SLJ or Jubilee or some other
\r?\n> I don't know about. Francophiles help!
\r?\n> Many thanks,
\r?\n> Robert Aguirre
\r?\n> Windsor, ON