The degradation of nebkha populations releases stabilized sediments, initiating a cascade of geomorphic, aeolian, and biological processes that push desert-oasis margins back into aeolian disasters. However, insufficient understanding of the vegetation-geomorphic dynamics of nebkha degradation hinders the simulation and prediction of ecosystem stability and landscape evolution after the loss of accumulation and barrier function in nebkhas. This study focused on collapsing-degraded Nitraria tangutorum shrub dunes (CDSDs) in two northwest China regions: Minqin County (between Tengger and Badain Jaran Deserts) and Yanchi County (southern Mu Us Sandy Land). By comparing vegetation patterns across dune patch scales, we inferred landscape evolution during degradation. At the individual dune scale, degradation shrinks the primary surface (PS), forms collapsed surfaces, and develops an accumulated surface (AS), driving the sudden and gradual decline of the N. tangutorum population and the loss of its constructive species status. The AS serves as a convergence zone for water, nutrients, and seeds, becoming a critical area for vegetation re-establishment. At the population scale, large-scale PS to AS transformation shifts landscapes from "fixed/semi-fixed sandy land" to "mobile sandy land", with pathways determined by interdune backgrounds. Where interdune area is fixed sandy land (Yanchi County), the reconstructed and stabilized AS facilitates the establishment of perennial herb assemblages (e.g., A. cristatum + S. alopecuroides association), ultimately driving the landscape transition into a gentle desert steppe. In contrast, under a semi-fixed sandy land background (Minqin County), vegetation on the AS is subject to strong wind erosion or sand burial and is dominated by short-lived psammophytes (e.g., A. squarrosum + S. tragus association). In particular, extreme climatic events can eliminate vegetation on the AS, converting the regional landscape into mobile sandy land. Therefore, restoration strategies must be context-specific, enclosure and natural succession in fixed sandy lands, and active intervention in semi-fixed lands to break the "dune activation-vegetation decline-aeolian intensification" cycle.