Per http://fedoraproject.org/wiki/Extras/FedoraDesktopEntryGuidelines: The vendor prefix (desktop-file-install --vendor=...) must be set to fedora".
I don't understand the rationale/motivation behind requiring '--vendor fedora'
I can, however, see that desktop-file-install's current implementation(*) of prepending %{vendor}- to the .desktop file name has some problems/issues: 1. .desktop filename now varies from upstream 2. --vendor may change when/if Extras bits are pulled into Core and/or RHEL. 3. *In particular*: when users start employing menu editors, since most(all?) of them base their customizations on the .desktop file name.
-- Rex
(*) If desktop-file-install's implementation instead followed something like kde's practice of using a vendor directory (e.g. /usr/share/applications/kde), then (1) and (3) would no longer be an issue.