Vacuum-deposited perovskite light-emitting diodes (PeLEDs) are attracting increased attention owing to their precise thickness control and absence of solvent-orthogonality constraints, offering significant potential for optimizing device performance. Here, we systematically compare how varying the deposition sequence of a single additive, triphenylphosphine oxide (TPPO)─known for its effective defect passivation─critically affects the crystallization dynamics, film morphology, and optoelectronic properties. Two distinct deposition strategies were compared: Co-passivation (simultaneous deposition of CsBr, PbBr2, and TPPO) and sequential-passivation (alternating ultrathin TPPO layers and perovskite layers). While Co-passivation delayed crystallization until annealing, sequential-passivation enabled partial crystallization during deposition, leading to smoother, more uniform films with higher photoluminescence quantum yield. Moreover, we demonstrate that TPPO induces quasi-2D perovskite formation, and to the best of our knowledge, this is the first report showing that a nonamine-based organic molecule induces quasi-2D formation. As a result, sequential-passivation devices achieved a higher external quantum efficiency (EQE) up to 10.9% and enhanced operational stability (T50 = 44 min) compared to Co-passivation devices (EQE = 7.4%, T50 = 16 min). This study highlights the importance of additive deposition sequence in determining the crystallization mechanism and optoelectronic properties of perovskite films, providing insights for designing high-performance vacuum-processed PeLEDs.