Vespa and Neo4j are two popular vector databases with different strengths. Vespa is a powerful search engine and vector database that can handle multiple types of searches, including vector search, text search, and structured data search. It's great for big data applications and supports tensor-based search. On the other hand, Neo4j is designed for graph data where relationships are as important as the nodes themselves. Its vector search capabilities combined with graph traversal make it a great option for developers who want to add semantic similarity matching to traditional graph queries. When choosing between them, consider your use case requirements, your data, and performance requirements of your application.