You likely will have to install the luggage rack. They are quite sturdy. If you are not good at fabricating stuff, it might be more difficult, but yes, the idea is to bolt the Givi base to the luggage rack..somehow. You might need to buy a larger "plate" to go inbetween the base and the rack as the rack is kinda small.
You might be able to retro fit the one from the 650 somehow, as it appears to have its own "side rails".
The one problem I see with most top cases fitted to the Vulcan is that it covers up alot of the tail light. From way behind this may not be a problem , but it does make it harder for cars right behind you to pick out your lights.
So what ever you come up with I think it would be a good idea to mount the case as high as you can.