Okay, well, let me speak from experience. I drive a Vespa LX150. I have gotten my scooter to 75mph. Here in Arizona, most people drive 70–75mph on the freeway. Your question was exactly what I asked the DMV person when I got my license. She told me “as long as you can safely keep up with traffic, you can drive on the freeway.” Since then, I will drive on the freeway every now and again when necessary but only when necessary. It does take some experience to be able to ride that fast especially when taking a curving offramp. About wind gusts, we have monsoons here. It takes a little getting used to, but have never been pushed to another lane even with 80mph gusts.
The GT200 and 250 models will go 85–90 and are hence safer.
About the whole comment of getting a bike that’s 350cc or higher, if you want to do frequent 75mph longer distance riding you may appreciate that more. Vespas are not about simply ‘riding a bike’ but also about the retro image of driving a European (Italian) metropolitan vehicle made by Piaggio. They are very efficient, and have long-lasting engines. As for other scoots out there, I vouch for none.