The introduction of defects, such as vacancies, into InxGa1-xAs can have a dramatic impact on the physical and electronic properties of the material. Here we employ ab initio simulations of quasirandom supercells to investigate the structure of InxGa1-xAs and then examine the energy and volume changes associated with the introduction of an arsenic vacancy defect. We predict that both defect energies and volumes for intermediate compositions of InxGa1-xAs differ significantly from what would be expected by assuming a simple linear interpolation of the end member defect energies/volumes.