  The Very Large Area gamma-ray Space Telescope (VLAST) is a mission concept
proposed to detect gamma-ray photons through both the Compton scattering and
electron-positron pair production mechanisms, enabling the detection of photons
with energies ranging from MeV to TeV. This project aims to conduct a
comprehensive survey of the gamma-ray sky from a low Earth orbit using an
anti-coincidence detector, a tracker detector that also serves as a low energy
calorimeter, and a high energy imaging calorimeter. We developed a Monte Carlo
simulation application of the detector with the GEANT4 toolkit to evaluate the
instrument performance including the effective area, angular resolution and
energy resolution, as well as explored specific optimizations of the detector
configuration. Our simulation-based analysis indicates that the VLAST's current
design is physically feasible, with an acceptance larger than 10~$\rm m^2\ sr$
which is four times larger than Fermi-LAT, an energy resolution better than 2\%
at 10~GeV, and an angular resolution better than 0.2 degrees at 10~GeV. The
VLAST project is expected to make significant contribution to the field of
gamma-ray astronomy and to enhance our understanding of the cosmos.
