We present a direct growth approach for h-BN/graphene heterostructures under controlled conditions using molecular beam epitaxy. The comprehensive growth study (covering various temperatures and growth times) reveals the evolution of 2D h-BN layers during van der Waals epitaxy on graphene from nucleation to coalesced layers. As a substrate high quality epitaxial graphene on SiC was selected for its good reproducibility. A detailed investigation using various correlated atomic force microscopy (AFM) techniques as well as Raman and X-ray photoemission spectroscopy revealed a nucleation which is mediated by morphological defects (wrinkles and step edges) and point defects in epitaxial graphene from where the 2D h-BN layers expand laterally (as a monolayer) and vertically (as multiple layers). Therefore, this study helps understanding the nucleation behavior during van der Waals epitaxy at elevated temperatures, which is a first step towards a controllable and scalable fabrication of 2D heterostructures.