What is the difference between RAG and a vector search system?

Vector search is one component inside a RAG system, not a synonym for it. A vector search system finds the most similar items to a query vector and returns ranked results, typically used for semantic search, recommendation, or deduplication. RAG uses vector search as its retrieval step but adds a language model that consumes the retrieved documents and generates a natural-language answer. You can run vector search without RAG, but you cannot run RAG without some form of retrieval, and vector search is the most common choice.