https://vocab.org/vann/#preferredNamespaceUri defines it merely as AnnotationProperty and doesn't specify Data vs Object prop, but all ontologies I know use vann:preferredNamespaceUri as a string not URL.
Eg see BIO (https://vocab.org/bio/schema.rdf) from the same author as VANN: <vann:preferredNamespaceUri>http://purl.org/vocab/bio/0.1/</vann:preferredNamespaceUri>
https://vocab.org/vann/#preferredNamespaceUri defines it merely as AnnotationProperty and doesn't specify Data vs Object prop, but all ontologies I know use
vann:preferredNamespaceUrias a string not URL. Eg see BIO (https://vocab.org/bio/schema.rdf) from the same author as VANN:<vann:preferredNamespaceUri>http://purl.org/vocab/bio/0.1/</vann:preferredNamespaceUri>https://github.com/Accord-Project/aec3po/blob/5fb6f7e2b8b9f0f208caeeab68461aa8b3c0dfcd/src/aec3po.ttl#L215