This study presents a thermodynamic modeling framework for optimizing phosphorus recovery from wastewater through the crystallization of vivianite Fe₃(PO₄)₂·8H₂O. With phosphorus identified as a critical raw material in the European Union and global phosphate reserves under geopolitical stress, recovering phosphorus from wastewater has become both an environmental and economic priority. The methodology involves constructing a thermodynamic model that integrates heterogeneous chemical equilibria, including hydrolysis, complex formation, ligand protonation, and mineral precipitation. The model evaluates interactions among phosphate, iron(II) ions, and potential complexing agents across a pH range of 7 to 11. Gibbs free energy ∆G^overall changes for the overall crystallization–dissolution process including competing reactions are calculated to define the thermodynamic stability areas for vivianite and co-precipitates such as hydroxyapatite, calcium phosphates, and iron hydroxides. Species distribution diagrams for the analyzed heterogeneous systems and overall Gibbs free energy ∆G^overall stability maps were constructed based on both synthetic and real wastewater compositions. Obtained thermodynamic results show that phosphorus recovery efficiency improves with Fe:P molar ratios between 1.5 and 1.8, with optimal crystallization occurring at pH 7.0 to 7.5. At higher pH values, the formation of competing mineral phases reduces the selectivity for vivianite. The model also provides quantitative predictions of residual phosphate and iron concentrations after the wastewater treatment. The developed thermodynamic framework facilitates improved control of vivianite crystallization parameters and supports nutrient recovery strategies aligned with circular economy principles. This model is adaptable to a range of wastewater types and informs the design of efficient, low-impact phosphorus recovery technologies.