Thank you all for the answers!
The reason my friend believes this vase has been reproduced is that it has a few layers of glass. He sells opalescent glass as a dealer and assures me it could not be so. He's telling all d'Avesn, Sabino, Lalique, Verlys's pieces must be colored in glass mass. Actually I'm collecting colored multicouche glass such as Daum, Schneider, Paul Nicolas etc and for me a vase blown to a mold having 1-2-3 or more layers is not a fake or reproduction. But I'm not a specialist in Opalescent glass that's why I'm looking for the answers.
I have browsed a lot of auction's sites where this vase was presented or sold and everywhere it looks the same - with this glaze of colorless glass covering the inner turquoise layer.
I also wonder about the signature. Did d'Avesn repeat this signature at his late reproductions? Is there any catalogue or resonnee where all his pieces are attributed, dated and gathered?