Scanning tunneling microscopy observations of the growth of Cl-induced etch pits on Si(100)-2 X 1 lend themselves to quantitative determinations of vacancy size distributions, particularly those that are linear or branched at solace point along the Linear pit. These result in rate constants and energy differences for the dominant etching channels. By considering nearest-neighbor and second-nearest-neighbor interactions, we determine the attractive interaction energies between dimers in a row 0.28 eV and between dimers in adjacent rows, 0.22 eV, as well as the repulsive second-neighbor dimers, -0.08 eV. These results are tested by means of Monte Carlo simulations.