Circular dichroism line shape functions are derived for a system consisting of two excited electronic states coupled to a single harmonic vibrational mode. A microscopic quantum field representation is used to develop the time-dependent one-exciton Green's functions for two different models of optical activity: the one-electron model and the coupled oscillator model. An analytic form of the Green's functions for these models is derived using an approximate diagonalization in the strong coupling limit of a general two-level exciton-phonon Hamiltonian [Friesner and Silbey, J. Chem. Phys. 75, 5630 (1981); Friesner, ibid. 76, 2129 (1982)]. Simulated spectra are presented which demonstrate differences between the coupled oscillator and one-electron mechanisms. Possible applications and extensions of this theory are discussed.