That's a beautiful Vespa. P200E? I also ride a Stella, and found from experience leaving stop lights with the front wheel in the air that you need to avoid putting much more weight in the back. Almost all the weight of both the scooter and the rider is already on the back wheel. I would put the heavier stuff on a front rack.
I just received a Shad topcase, along with a bike specific luggage rack/mounting base for my Zuma 125. I was going to get the Yamaha rack, and rig a top box on it, but found Shad had everything. Their rack was made specifically for their mounting bases. No making homemade brackets. I got a 33 liter top case. With the top case, underseat storage (which the Vespa and Stella don't have) a backpack, and using the floorboard for storage (I also got highway pegs) it should be enough room for a several day trip. I may add soft saddlebags later.
You are almost certainly looking at an adventure. At my age and in my condition I would not take off on a long trip on an old 2 stroke Vespa or a Stella. They definitely have the look and character I love, but they are not the most reliable scooters ever made. And while I have done it, I am no longer in any condition to be rebuilding an engine beside the road.
I hope you make a trip report on it.
2002 Vulcan 750 (engine out, slowly being disassembled) 2013 Royal Enfield B5
2001 XT225, 2009 Genuine Stella
2012 Zuma 125, 1980 Puch moped