The engineering of blood vessels that could ensure efficient transport of various nutrients and metabolites is a challenge in tissue engineering. The creation of cell-seeded bioconstructs using modified natural polymers, in particular, PEGylated fibrin is under investigation, which will help overcome this problem. Therefore, the purpose of this study was to determine the optimal ratio of the hydrogel components of modified fibrin to provide favorable conditions for the vascular development of endothelial and mesenchymal stem cell coculture. We have shown that PEGylated fibrin gels are capable of maintaining three-dimensional growth of HUVEC and hASC cells. Hydrogel with a filamentous microporous structure obtained from PEGylated 5 : 1 fibrinogen and thrombin at a concentration of 0.2 U per 1 mg ensured optimal conditions for spreading, growth, and development of cocultured cells as well as the expression of proteins involved in angiogenesis.