Blending a keyword index into vector search is the default advice, and the averages back it: the best blend beat vector-only retrieval on 33 of 35 encoder-and-corpus pairs. Scored one query at a time, the same blend ranked the relevant document below whichever retriever was better alone on 13–48% of queries, and below plain vector-only on 8–21% — for an average gain of 0.008–0.051 on a 0-to-1 ranking score. Which queries pay is close to unguessable, so the decision has to be made per corpus, from a measurement, before you blend.