Using density functional theory calculations, we investigate the microscopic origins of the distinct ground states in monolayer $T\text{−}{\mathrm{VSe}}_2$ and $H\text{−}{\mathrm{VSe}}_2$. We reveal that $T\text{−}{\mathrm{VSe}}_2$ exhibits competing tendencies towards either ferromagnetic (FM) or charge density wave phases, governed by the Fermi surface nesting in the weak-coupling limit. In contrast, $H\text{−}{\mathrm{VSe}}_2$ stabilizes a robust FM ground state within the local moment picture of the strong-coupling limit. The dichotomy originates from contrasting correlation strengths due to distinct orbital degrees of freedom controlled by the crystal field splittings between $T\text{−}{\mathrm{VSe}}_2$ and $H\text{−}{\mathrm{VSe}}_2$. We argue similar physics can also be applied to ${\mathrm{VS}}_2$ and ${\mathrm{VTe}}_2$ in the T and H phases. While triply degenerate $t_{2g}$ orbitals with a larger bandwidth favor the itinerant scenario in the weak-coupling limit in the T phase, a nondegenerate $d_{z^2}$ orbital with a narrower bandwidth supports the local moment picture in the strong-coupling limit in the H phase. Furthermore, we demonstrate the tunability of these states via Se height, offering pathways to manipulate quantum phases in $T\text{−}{\mathrm{VSe}}_2$. This work provides a comprehensive understanding of various conflicting experimental findings and theoretical predictions regarding polymorphic monolayer ${\mathrm{VSe}}_2$.

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• Charge density waves
• Density of states
• Fermi surface
• Magnetic coupling
• Electronic structure
• Ferromagnets
• Transition metal dichalcogenides
• Density functional theory
• Heisenberg model
• Tight-binding model
• Wannier function methods