  We associate vertex operators to space-time diffeomorphisms in flat space
string theory, and compute their algebra, which is a diffeomorphism algebra
with higher derivative corrections. As an application, we realize the
asymptotic symmetry group BMS3 of three-dimensional flat space in terms of
vertex operators on the string worldsheet. This provides an embedding of the
BMS3 algebra in a consistent theory of quantum gravity. Higher derivative
corrections vanish asymptotically. An appendix is dedicated to alpha prime
corrected algebras in conformal field theory and string theory.
