Large language models (LLMs) and agentic AI are shifting software engineering from manual coding toward intent specification, architecture, and governance. Two paradigms have emerged: vibe coding, an intuition-driven approach accepting AI artifacts via observed behavior, and Specification-Driven Development (SDD), which uses structured specifications as the authoritative source of truth. This article makes three contributions. First, based on a verified literature corpus, it identifies failure modes of ungoverned conversational generation: the productivity-reliability paradox, architectural erosion from limited context, security exposure, and technical debt. Second, it introduces the Specification Governance Reference Model (SGRM). This tool-independent framework defines four-component specification contracts, constrains stochastic generation via deterministic validation, formalizes three rigor levels, and integrates generation, verification, and governance into a closed-loop architecture. Third, it evaluates SGRM against ISO/IEC 25010, mapping quality characteristics to governance mechanisms. Empirical evidence supports this, reporting a 73% reduction in security defects under constitutional constraints and a 50% reduction in time-to-market through specification-governed agentic delivery. The analysis concludes that while vibe coding is valuable for ideation and rapid prototyping, enterprise software requires specification governance to transform probabilistic AI generation into deterministic, auditable engineering. Boundary conditions, threats to validity, and future research directions are discussed.

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