The Steinberg VST3 SDK is dual-licensed under a proprietary license or GPLv3, at the user’s choice (https://developer.steinberg.help/pages/viewpage.action?pageId=9797944). So far, so good. There are some files in the tarball that are not dual-licensed (https://developer.steinberg.help/pages/viewpage.action?pageId=9797948), and these would need to be removed to create a redistributable version of the source archive—again, a common and well-documented situation.
However, there is another wrinkle. Steinberg imposes a long list of trademark-related “usage guidelines” (https://developer.steinberg.help/display/VST/Steinberg+VST+usage+guidelines) on all uses, including those under the GPLv3 license option.
I fear these additional terms may make the VST3 SDK unsuitable for packaging in Fedora. If so, the subset of the VST3 SDK bundled in the giada package (which I recently started maintaining) may need to be removed as well. However, I would appreciate input from this list before proceeding in either direction—either removing the bundled SDK (hopefully doing so in %prep would be sufficient) and disabling the associated optional functionality in Giada, or attempting to unbundle the SDK as a separate Fedora package.