We report successful synthesis of a mixed V/Fe ultra-thin oxide phase on Ru(0001), in which mixing is governed by a self-organization scheme that selects V/Fe compositions close to 50%. Bias-dependent high resolution STM imaging and DFT simulations enabled a thorough characterization of its honeycomb-like structure with additional oxygen anions at the metal/oxide interface, as well as a convincing assignment of the two observed defect types. The mixed layer displays a unique structure and stoichiometry intermediate between those of pure vanadium and iron oxide films under the same experimental conditions. It results from a singular, geometry-driven mixing scenario in 2D which, thanks to the high interfacial oxygen content, is reminiscent of that known for bulk ternary oxides. Comparison with V/Fe mixed honeycomb layers on Pt(111) exemplifies how the choice of the metal substrate may induce largely different mixing behaviors. Such versatility opens a way towards a thorough fundamental understanding of the principles that underlie oxide alloying in 2D.