Medin, a vascular amyloid derived from MFG-E8, is the most prevalent form of localized human amyloid and co-localizes with Aβ in Alzheimer's disease and, in particular, cerebral amyloid angiopathy (CAA). While it was shown that medin can promote Aβ aggregation, it remains unclear whether this amyloid-amyloid interaction affects the structure of the resulting fibrils. Here, we investigate how medin modulates Aβ40 fibril assembly in vitro using cryo-electron microscopy, aggregation kinetics, and immunogold electron microscopy. We show that medin accelerates Aβ40 aggregation, co-assembles into hybrid fibrils, and modulates fibril morphology. Cryo-EM analysis reveals two fibril populations: one corresponding to a previously described in vitro Aβ40 morphology, and a second, previously unobserved polymorph with Aβ42-like features, including a structured N-terminus and a compact hydrophobic C-terminal core. The presence of a peripheral, unresolved cryo-EM density near the fibril surface suggests that the new polymorph is stabilised through heterotypic interactions, yet the atomic details remain unresolved, likely due to substantial structural heterogeneity. Rather than representing a limitation, this highlights how not all determinants critical for fibril assembly are necessarily ordered or resolvable in the final fibril structure, reflecting the inherent dynamic and heterogeneous nature of amyloid interactions. Our findings provide structural evidence that heterotypic co-aggregation can redirect Aβ40 into distinct conformational states and suggest that dynamic or transient interactions contribute to fibril polymorphism beyond what can be fully captured in static structural models.