I've got a GTS Touring (/w luggage rack), and I want to start touring on it.
I ride in California, so I don't want to make it any wider, and I'm skeptical that I could comfortably store a multinight trip's worth of stuff in the tunnel between my knees. It seems like my best option is to strap a bag either to the pillion or luggage rack, depending on how much it weighs.
I could get a motorcycle-specific bag like the Kurykakyn Torke, or I could get a backpack like the Patagonia Cragsmith and some ROK straps to keep it secured.
The Cragsmith is nice because the back zips off, so you can see everything in your bag without digging. My current backpack becomes a mess on a long trip, because you can't see the bottom of it when the zipper only goes halfway down. As a backpack, the Cragsmith also seems more versatile: easier to take on non-moto trips, and to carry when the Vespa is parked.
Is there a reason to prefer motorcycle-specific luggage? It would be more waterproof, although I can't imagine riding in rain heavy enough to care. It's also designed to be secured to a bike, but I don't see why I couldn't just attach some ROK straps to a regular bag, secured to either the grab rail or luggage rack.