Vector index tuning is the specialized art of optimizing approximate nearest neighbor (ANN) indices in vector databases to balance search speed, recall accuracy, and memory consumption. Advanced tuning goes beyond default configurations: it involves profiling query latency distributions, analyzing recall-precision curves, and strategically adjusting parameters like ef, M, and quantization settings based on production workload patterns. This skill encompasses both algorithmic understanding (HNSW, IVF, LSH, Product Quantization) and hands-on optimization of platforms like Pinecone, Milvus, Qdrant, and open-source FAISS. Vector indices power semantic search, recommendation engines, and RAG (Retrieval-Augmented Generation) systems at scale. A poorly tuned index can waste 10x memory or add 500ms to latency; expert tuning cuts that to single-digit milliseconds with minimal footprint. Organizations running billions of embeddings depend on this skill to keep search latency sub-100ms while keeping recall >90%.