Mount vendor? You mean ... using the mount instead of walking around?
Open up your Honorbuddy folder.
Open up the Settings Folder
Navigate to the realm your character is located on (example: Mal'Ganis)
Open your character's folder
Open the CharacterSettings.xml in notepad.exe (or equivalent editor)
Search for: <UseMount>True</UseMount>
Change from True to False
Hope that helps.
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I re-read the question and re-considered my reply.
If you're talking about "repair mount" (Traveling Mammoth/Traveling Yak) for selling/repairs, etc, instead of moving to a dedicated area (preset by the profile), I haven't yet figured out where that is stored. I'm still looking, momentarily. Firstly, I'd ensure that there is a pre-set "move to for repairs/item sale/mail" within the farming profile.