VexDB-Lite is a C++ vector similarity search engine distributed as embedded database extensions for PostgreSQL, DuckDB, and SQLite, with additional iOS and WebAssembly demo surfaces. The core implements an HNSW-like graph index, product quantization (PQ), RaBitQ binary quantization, and a large SIMD distance-kernel layer spanning SSE, AVX, AVX512, NEON, and SVE. The PostgreSQL side is a full index access method with floatvector type, parallel build, WAL persistence, and vector buffer management; the DuckDB side plugs in as an extension with optimizer rewrites and physical index scans; the SQLite side provides a virtual table and scalar functions with segment-based disk persistence. This is an application repo, though it has significant research flavor in its ANN algorithms, quantizer implementations, and benchmark-driven validation methodology.