A simple and computationally tractable constitutive equation is developed for concentrated and entangled polymer systems, based upon a single vector description of the polymer configuration. The constitutive equation is intended to mimic the DEMG model, and allows the vector to stretch and orient on different time scales. The constitutive equation is shown to yield steady material functions in homogeneous, extension-dominated flows that are in excellent qualitative and quantitative agreement with the DEMG model. The constitutive equation is shown to exhibit less shear thinning than the Doi-Edwards reptation model, and therefore also shows a shear viscosity in better agreement with experiment. In simple shear flow the model predicts chain stretch at large shear rates, in contrast to the DEMG model, but in qualitative agreement with recent experimental studies (Bercea et al., Macromolecules 26 (1993) 7095-7096). In start up of simple shear, the model predicts an undershoot in the orientation angle, and over; and undershoots in the birefringence in qualitative agreement with experiments.