Continual test-time adaptation (CTTA) adapts a pre-trained medical segmentation model online to an unlabeled target stream whose distribution changes over time. However, most existing CTTA methods rely on pseudo-labeling and self-supervised objectives, which inevitably yield noisy supervision under domain shifts. To mitigate this limitation, we introduce off-the-shelf Vision Foundation Models (VFMs) as external knowledge sources. Zero-shot VFMs are insufficient for medical segmentation because they lack medical semantics, yet they contain rich and heterogeneous generic knowledge. To exploit such external knowledge, we propose Reciprocal Distillation with a Frozen Foundation Model (ReDi-FM), a novel framework with two core components: using structure-aware prompts from the source model to guide the frozen VFM to generate target-adapted supervision, and distilling the resulting knowledge back into the adapting model. For robust distillation, we introduce two complementary objectives: uncertainty-driven hard distillation for precise guidance in ambiguous regions, and hard class-balanced soft distillation for richer supervision of under-represented and challenging structures. A consensus-aware gate further stabilizes adaptation when the two teachers disagree. Extensive experiments on multi-domain medical segmentation benchmarks demonstrate that ReDi-FM outperforms state-of-the-art CTTA methods. Code is available at https://github.com/M4cheal/ReDi-FM.