Surface functionalization strategies of synthetic materials for regenerative medicine applications comprise the development of microenvironments that recapitulate the physical and biochemical cues of physiological extracellular matrices. In this context, material‐driven fibronectin (FN) nanonetworks obtained from the adsorption of the protein on poly(ethyl acrylate) provide a robust system to control cell behavior, particularly to enhance differentiation. This study aims at augmenting the complexity of these fibrillar matrices by introducing vitronectin, a lower‐molecular‐weight multifunctional glycoprotein and main adhesive component of serum. A cooperative effect during co‐adsorption of the proteins is observed, as the addition of vitronectin leads to increased fibronectin adsorption, improved fibril formation, and enhanced vitronectin exposure. The mobility of the protein at the material interface increases, and this, in turn, facilitates the reorganization of the adsorbed FN by cells. Furthermore, the interplay between interface mobility and engagement of vitronectin receptors controls the level of cell fusion and the degree of cell differentiation. Ultimately, this work reveals that substrate‐induced protein interfaces resulting from the cooperative adsorption of fibronectin and vitronectin fine‐tune cell behavior, as vitronectin micromanages the local properties of the microenvironment and consequently short‐term cell response to the protein interface and higher order cellular functions such as differentiation.