Ahoy ahoy. So I was looking to do this as well for a profile I'm writing. I found the HB wiki pages to be very helpful, particularly the pages on custom behaviors. I successfully used a RunMacro behavior followed by a ForceSetVendor behavior, with only appropriate parameters included.
First things first, at the top of your questing profile under the <Vendors> tag, include the following line:
<Vendor Name="Gnimo" Entry="32639" Type="Repair" />
Second, and lastly, include these two custom behavior lines whenever it is in the profile that you would like to force your bot to summon his mammoth and repair/vendor:
<CustomBehavior File="RunMacro" Macro="/cast Traveler's Tundra Mammoth" />
<CustomBehavior File="ForceSetVendor" DoRepair="true" DoSell="true" />
This may not be exactly what you were looking for, but I hope it helps somewhat.