This paper proposes an integrated framework for staging and trajectory optimization of a launch vehicle under a constraint on the instantaneous impact point (IIP) of the separated stage. The proposed framework iterates between the optimal staging module and the trajectory optimization module to minimize the lift-off mass of a launch vehicle for a given payload mass. The trajectory optimization module finds the velocity losses and velocity change for IIP control, which flow to the optimal sizing module as inputs. The iteration between the two modules continues until the convergence criteria are satisfied. The effectiveness of the proposed framework has been demonstrated through a case study.