Is this bike Steel or Aluminium framed??
Most French brands used Simplex DOs if they wanted forged DOs on their bikes. I don't think Vitus DOs were that common to start with. The Solida crank seems to point it towards a mid level to lower model bike as Solidas were usually seen on such. A high model bike would have had at least a nice TA, Stronglight, Mavic or Campagnolo crankset....so I'm hoping that the Solida was a later replacement for a better one that might be originally on the bike. The Mafac dirlled levers were quite common, but I have seen different finish levels on them and they can be quite beautiful if you have the better ones. British BB seems to point to at least an 80's bike, but I think some 531 tubed French bikes might have English treaded BBs in the 70's too.
It would help to see good pitctures of the frame to get a better answer to what this frame is.
Chombi