This work demonstrates the atomic vacancy effects on the phonon properties of armchair silicene nanoribbon in a step by step process for the first time. The phonon localization effect figures out the fact that vacancies cause to high-energy phonons become localized, whereas low-energy phonons can easily transmit. The vacancy reduces high-energy phonon transmission severely compared to low-energy phonon. It is also found from phonon density of states that high-frequency phonons soften towards the low-frequency region. The simulated phonon bandstructure verifies that most of the phonon branches transform to a nondegenerate state from a degenerate state and shifted toward a lower frequency regime due to the presence of vacancies. The overall consequences of atomic vacancies on the phonon thermal conductance disclose the reality that only a few atomic vacancies result in a vital reduction of phonon thermal conductance. In addition, the entropy of the disordered system is investigated.