I'm afraid I can't agree with tony_dt.
While I have no right to respond to this question since I don't own one yet, when you consider the fact that KYMCO advertises the Grand Vista as, "KYMCO's grand vision of how well appointed a touring scooter should be." I would certainly hope that it would be interstate capable since that is implied in the name, Touring Scooter. That's one of the things that I'm buying it for.
Tony_dt's comment, "acceleration from 60-80 MPH is not going to be near what a larger bike can produce." doesn't make a lot of sense to me either since most interstate travel is at a steady speed, mile after boring mile. How long do you think it takes a semi-truck to get from 60-80 (or for that matter from 0-60)? I don't like interstate travel. I prefer roads that follow the land, not cut through it. (OK, so I plagiarized that from the movie, "Cars". That doesn't make it any less true.
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As for, "You have to be a defensive driver and keep an eye on traffic." that's true no matter what you drive or ride never so much as when you're on two wheels. At least it's true if you intend to become an old scooterist.
Life is not a destination, it's a journey.