vSphere Update Manager refused to import the ISM-Dell-Web-2.3.0-223.VIB-ESX60i_A00.zip - saying it's an invalid package.
Turned out the vendor-index.xml inside the zip file has duplicated <metadata> section and missing a version number in the <version> tag. Unzipped the file, modify the file, pack it back to a zip. Then it'll be imported okay.