Carrier transport in GaAs-based vertical-cavity surface-emitting lasers (VCSELs) is investigated by means of an in-house multiphysics code, with particular emphasis on the description of many valley effects in the conduction band of AlGaAs barriers. These effects, which are revealed to have a significant impact on the overall VCSEL performance, are accounted for by an effective density of states obtained with a closed-form model. This description has been included in a simplified simulation framework, where most of the distributed Bragg reflector pairs are replaced by an equivalent homogeneous layer. This leads to a major reduction of the computational cost, especially important in view of the computer-aided design of 3D devices.