Molecular crosstalk between intra-tumor blood vessels and tumor cells plays many critical roles in tumorigenesis and cancer metastasis. However, it has been very difficult to investigate the biochemical mechanisms underlying the overlapping, multifactorial processes that occur at the tumor-vascular interface using conventional murine models alone. Moreover, traditional two-dimensional (2D) culture models used in cancer research do not recapitulate aspects of the 3D tumor microenvironment. In the present study, we introduce a microfluidic model of the solid tumor-vascular interface composed of a human umbilical vein endothelial cell (HUVEC)-lined, perfusable, bioengineered blood vessel and tumor spheroids embedded in an extracellular matrix (ECM). We sought to optimize our model by varying the composition of the tumor spheroids (MDA-MB-231 breast tumor cells + mesenchymal stem cells (MSCs)/human lung fibroblasts (HLFs)/HUVECs) and the extracellular matrix (ECM: collagen, Matrigel, and fibrin gels with or without free HLFs) that we used. Our results indicate that culturing tumor spheroids containing MDA-MB-231 cells + HUVECs in an HLF-laden, fibrin-based ECM within our microfluidic device optimally (1) enhances the sprouting and migration of tumor spheroids, (2) promotes angiogenesis, (3) facilitates vascular invasion, and (4) preserves the structural integrity and functionality of HUVEC-lined microfluidic channels. This model may provide a platform for drug screening and mechanism studies on solid tumor interactions with functional blood vessels.