Spin defects in two-dimensional materials are promising platforms for quantum information technologies and nanoscale sensing. The negatively charged boron vacancy ( ${\mathrm{V}}{\mathrm{B}}^{-}$ ) in hexagonal boron nitride (hBN) is particularly attractive due to its sensitivity to temperature, magnetic fields, and pressure, but its application is limited by intrinsically weak photoluminescence (PL). Here we demonstrate a van der Waals heterostructure that enhances the PL intensity of ${\mathrm{V}}{\mathrm{B}}^{-}$ by up to 45× at 4 Kelvin while preserving compatibility with ultrathin optoelectronic platforms. A quasi type-I band alignment at the PbI 2 –defected hBN interface facilitates efficient interlayer coupling to the defect, and strong spectral overlap supports fluorescence resonance energy transfer. Ab initio calculations indicate that this coupling enables a photon-ratcheting mechanism, in which optical absorption in PbI 2 generates a nonlinear polarization that drives ${\mathrm{V}}_{\mathrm{B}}^{-}$ excitation. This process is compatible with interlayer energy transfer while maintaining minimal electronic and spin hybridization and preserving optically detected magnetic resonance (ODMR) contrast. The heterostructure further exhibits improved sensitivity to external magnetic fields. These results establish a route for the enhancement of defect emission via interlayer optical coupling and introduce a general materials strategy for amplifying weak quantum defect signals in nanomaterials.