I traded my VTX 1800 for a Vee. I had the same questions you did about how my back would react to the new riding position. I was also wondering if I would put more pressure on my hands causing my hands to numb up.
After driving off with my Vee (a little teary-eyed at leaving my baby that I raised from a naked VTX into a distance-cruising monster:bom_bigcry
I drove around town and put 50 miles or so on the bike. My Vee has risers so my riding position is fairly upright causing no pain. The upright position also reduces the pressure on my hands, but I did feel a bit of numbness until I fine-tuned my position. Another plus for me is that the previous owner installed electronic cruise control. Yeah, I like that...
Still not fully convinced that I had made the right decision I took the Vee around Flaming Gorge Reservoir in Wyoming and Utah with a side-trip to Vernal Utah to ride the switchbacks. This is about a 250 mile trip and was no problem on my VTX. I found the Vee to be fairly comfortable, easy on my back and hands though I was wishing for the pegs to be a bit lower - I can deal with that later. I also think I have an aftermarket seat though I haven't looked to be sure.
What I noticed more about the Vee was how much more fun it was to ride than my VTX. With a cruiser you are easing into turns, almost as though you are a passenger on the bike. With the Vee (and other upright bikes) you are attacking the corner, showing it that you are the boss, even if you aren't scraping pegs. I just enjoyed the ride much more on the Vee.
I don't know how far I can go in one sitting because I kept stopping to get pictures of the bike here or there.
Most of the few problems I have with the Vee can be fixed with some farkling (is that even a word?). The one major problem I have is that when I get within 50 yards of a semi on the interstate (75 mph or above) the turbulence rocks the handlebars all over the place. This is easily solved by riding behind the side of the truck so I can see what's ahead of it and passing at first opportunity (I learned that with my KLR), but still a bit annoying at times.
For any problem you have with the Wee you should decide if that is a problem that can be fixed by swapping out parts or not.If the handlebars are too low, buy a riser. If the seat hurts, buy a different seat. Don't let little things get in the way of the big picture. On the other hand, if you don't like the upright riding position, you may want to consider a different bike.
Just my $0.02