Photoluminescence spectroscopy was employed to probe the nature of optical transitions involving Al vacancy (V Al) and vacancy-oxygen complex (V Al-O N) in AlN. An emission line near 2 eV due to the recombination between the 2- charge state of (V Al-O N) 2-/1-, and the valence band was directly observed under a below bandgap excitation scheme. This photoluminescence (PL) band was further resolved into two emission lines at 1.9 and 2.1 eV, due to the anisotropic binding energies of V Al-O N complex caused by two different bonding configurations-the substitutional O N sits along c-axis or sits on one of the three equivalent tetrahedral positions. Moreover, under an above bandgap excitation scheme, a donor-acceptor pair like transition involving shallow donors and (V Al-O N) 2-/1- deep acceptors, which is the "yellow-luminescence" band counterpart in AlN, was also seen to split into two emission lines at 3.884 and 4.026 eV for the same physical reason. Together with previous results, a more complete picture for the optical transitions involving cation vacancy related deep centers in AlGaN alloy system has been constructed.