The Fisher cluster model provides a phenomenological description of the populations of molecular clusters in a vapor. However, it was not designed for use at all cluster sizes, including monomers, although this is often assumed to be the case when constructing models of droplet nucleation. In order to study the accuracy of this assumption, a set of models is constructed in which, progressively, the monomer, dimer, and trimer populations are described using empirical virial coefficients, while the Fisher model accounts for higher clusters. The models are used to study the populations at the critical temperature, which is an important reference point for model parametrisation. Remarkably, the cluster populations are little changed by altering the crossover point between the virial and Fisher expressions, and furthermore, the original Fisher model provides a reasonable description even for monomers, as desired. The model is extended in one of a number of possible ways to describe cluster populations at temperatures below the critical point with less success : the Fisher model can then no longer account for the whole size distribution.