Quasi-soliton modelocking has been identified as the mechanism responsible for the formation of picosecond pulses in passively mode-locked VECSELs, but neither this mechanism nor Kerr lens modelocking can account for the formation of sub-picosecond pulses from these lasers. Numerical simulations have shown that the optical Stark effect is capable of shortening pulses in the absence of bleaching, but to date no studies have been performed under realistic operating conditions. We model the interaction of an optical pulse with an absorbing quantum well using a semi-classical two level atom approximation. As the bandwidth of a VECSEL pulse is small compared to the spread of energies within a semiconductor band the population of two level atoms is divided into "live" atoms which interact with the optical field, and "dead" atoms which do not. Live and dead states are coupled by carrier-carrier scattering. Results from this model show an increase in pulse shortening above that due to saturable absorber bleaching at pulse durations below one picosecond, implying that an additional effect is responsible for the formation of femtosecond pulses. At these pulse durations the model predicts that the absorbing resonance broadens and decreases in amplitude. This is recognisable as a result of the optical Stark effect. The predictions of this model are compared to experimental results from several femtosecond VECSELs. For some modelocked VECSELs an excellent match between simulation and experiment is found, but in other cases the model cannot reproduce experimental results. We conclude that while the optical Stark effect may be the dominant pulse shaping mechanism in some modelocked VECSELs, others appear to be dominated by other effects.