Formamidinium Lead Bromide (FAPbBr₃) perovskite has drawn scientific interest in the field of optoelectronic devices. However, the presence of highly toxic lead restricts its application. We address here the role of Sn substitution and Pb vacancy (Pb-Vc) in regulating stability using state-of-the-art hybrid density functional theory. The explicit role of spin orbit coupling (SOC) and electron self interaction error are discussed. We find while SOC has a significant role in regulating electronic bands, it doesn't affect the relative hierarchy of thermodynamic stability of different configurations of FAPb_(1-x-y)Sn_xVc_yBr₃. Further, the co-existence of Sn substitution and Pb-Vc have been found to be thermodynamically stable via ab initio thermodynamics approach. However, Pb-Vc introduces trap states deep inside the forbidden gap, which leads to electron-hole recombination centers. Therefore, the stable configurations with only Sn substitution (FAPb_1−X Sn_X Br₃) are considered for further optical study. Because of the better optical properties, higher absorption coefficient, and larger spectroscopic limited maximum efficiency (SLME) values, the mixed perovskite FAPb_1−X Sn_X Br₃ with 0 ≤ X ≤ 0.5 are considered to be promising candidates for solar cell application.