Nanomechanical mapping of interfacial broadening between an epoxy network and a linear poly(methyl methacrylate) (PMMA) showed the generation of a new network resulting from transesterification reactions. The rate of interfacial broadening was found to decrease with annealing time and deviated significantly from that expected for a classical first-order transesterification reaction. The epoxy vitrimer/PMMA composites prepared by melt blending show a single-phase and homogeneous structure, quite different from thermoset and thermoplastic composites prepared by reaction-induced phase separation. The as-prepared composites have a dimensionally stable structure that does not decompose at elevated temperatures. Moreover, the stress relaxation of the composites slows with the vitrimer content and the stress does not relax even at extended times.