💡 The paper addresses the issue of modality sensitivity in vision-language models, which occurs when a model's performance degrades significantly when the modality of the input is changed, such as replacing a textual question with its rendered-image counterpart. This problem arises due to the inherent bias in current training corpora, where text and images are typically organized into distinct and asymmetric roles. To address this issue, the authors propose Local Modality Substitution, a data curation approach that provides supervision for cross-modal representational invariance between semantically equivalent text and image carriers. This method reformulates single-modality prompts into seamlessly interleaved multimodal sequences by dynamically selecting target text spans and recasting them as rendered images, thereby preserving the same semantics across different carriers. The authors evaluate their approach on 13 diverse multimodal benchmarks and demonstrate that it significantly improves overall multimodal reasoning and yields deeper cross-modal fusion, achieving consistent gains across foundational models. Specifically, the approach delivers improvements of 2.67 points on one model and 2.82 points on another, compared to standard methods. The proposed method is lightweight and architecture-agnostic, making it a valuable contribution to the field of vision-language models.