<p>Vapour deposition offers a scalable and industry-compatible route for perovskite light-emitting diodes, yet the process remains challenging owing to kinetically driven crystallization that produces mixed-dimensional phases and nanoscale heterogeneity. In particular, the lack of thermodynamic control leads to phase-disordered nanostructures, broadened energy landscapes and limited device efficiency. Here we report a thermodynamically guided vapour-phase synthesis of X-type quasi-two-dimensional perovskites, (CsPbBr₃)_<i>n</i>−1Cs₂PbBr₂X₂, with controlled nanoscale phase distribution and interfacial coherence. By introducing a halide-site-substituting organic spacer molecule that covalently binds to Pb²⁺ during in situ deposition, we achieve selective crystallization of quasi-two-dimensional phases with high phase purity. A self-assembled hetero-scaffold of LiF and spacer molecule acts as a nanoscale growth template, promoting spatially uniform nucleation and minimizing phase segregation. Multimodal structural and spectroscopic analyses reveal dimensionally and spatially homogeneous films with high photoluminescence quantum yield (&gt;85%) and reduced trap densities, enabling efficient exciton confinement and narrow emission. The resulting perovskite light-emitting diodes achieve an external quantum efficiency of 21.9%, an electroluminescence linewidth of 78.5 meV and operational stability exceeding 1,500 min, with scalable pixel arrays demonstrated. These results provide a scalable route to high-efficiency vapour-deposited perovskite optoelectronics.</p>