Museo de Arqueologia e Historia Francisco Fonck is Vina del Mar's most useful museum stop, especially if your Chile trip will not include Easter Island. The moai outside gets the photographs, but the interior collection ranges through Rapa Nui material, pre-Columbian archaeology, and natural history.
It is compact and a little old-fashioned in presentation, which is part of its charm. Visit when you want context between beach walks and garden strolls, and give yourself enough time to read rather than treating the moai as the whole attraction.