The process in which neuroepithelial cells in the neural tube acquire specialized structural and/or functional features of ventral spinal cord interneurons. Ventral spinal cord interneurons are cells located in the ventral portion of the spinal cord that transmit signals between sensory and motor neurons and are required for reflexive responses. Differentiation includes the processes involved in commitment of a cell to a specific fate. [ GOC:dgh http://purl.obolibrary.org/obo/go/references/0000021 GOC:cls GOC:dph GOC:jid http://www.ncbi.nlm.nih.gov/pubmed/11262869 ]