  In this paper, a new construction of vertex algebras from more general vertex
operators is given and a notion of quasi module for vertex algebras is
introduced and studied. More specifically, a notion of quasi local
subset(space) of $\Hom (W,W((x)))$ for any vector space $W$ is introduced and
studied, generalizing the notion of usual locality in the most possible way,
and it is proved that on any maximal quasi local subspace there exists a
natural vertex algebra structure and that any quasi local subset of $\Hom
(W,W((x)))$ generates a vertex algebra. Furthermore, a notion of quasi module
for a vertex algebra is introduced and it is proved that $W$ is a quasi module
for each of the vertex algebras generated by quasi local subsets of $\Hom
(W,W((x)))$. A notion of $\Gamma$-vertex algebra is also introduced and
studied, where $\Gamma$ is a subgroup of the multiplicative group $\C^{\times}$
of nonzero complex numbers. It is proved that any maximal quasi local subspace
of $\Hom (W,W((x)))$ is naturally a $\Gamma$-vertex algebra and that any quasi
local subset of $\Hom (W,W((x)))$ generates a $\Gamma$-vertex algebra. It is
also proved that a $\Gamma$-vertex algebra exactly amounts to a vertex algebra
equipped with a $\Gamma$-module structure which satisfies a certain
compatibility condition. Finally, three families of examples are given,
involving twisted affine Lie algebras, certain quantum Heisenberg algebras and
certain quantum torus Lie algebras.
