  We revisit the construction of deformed Virasoro algebras from elliptic
quantum algebras of vertex type, generalizing the bilinear trace procedure
proposed in the 90's. It allows us to make contact with the vertex operator
techniques that were introduced separately at the same period. As a by-product,
the method pinpoints two critical values of the central charge for which the
center of the algebra is extended, as well as (in the $gl(2)$ case) a Liouville
formula.
