I can't answer all of your questions, but I ride with someone who has a Vita. I see it as more of a road bike, actually, a flat bar road bike/hybrid than a mountain bike. I have a Jamis Coda as my second bike and I think the Vita is actually lighter and somewhat similar. The only way you answer your questions about the stand over is to get him on the bike.
Just one note. My son started mountain biking at 13. He got a road bike at 14 and pretty much taco'd the wheels like the first week because he thought he could jump the curbs, etc., like he did with the mountain bike. That was an expensive lesson. So your nephew might need a warning about no popping wheelies with his new bike.
It's great that you are helping your nephew get into road cycling. Not sure about the tandem. Does he want to do that? That would severely limit him to riding only when auntie can ride.