I'm afraid that you're right, Tim. If you install Win 10 from the web onto a thumb drive or DVD, it is the same one that people are downloading for free. It will go onto a computer with Vista or even XP, if the hardware supports it. But it will check for a legal 7, 8, or 8.1 key, and refuse to activate if it doesn't find one.
You can install the Win 10 preview over Vista, and MAYBE it will continue to run well for a while.
People who installed a legally-purchased Windows 8 Upgrade over the free preview version were technically allowed to do so, but later, MS started harassing them with pop-ups, and then would not let them install 8.1 or 10.
Getting back to Vista, you can pay $119 for a legal Windows 10 that will only do a clean install. But the old hardware will have a hard time with it. You would probably do better buying a used 7 or 8 computer and installing 10 for free. Incidentally, the used Windows 8 laptop I bought from a pawnshop installed 10 with no problem.
In my opinion, people still using XP or Vista are getting what they want (They can't play modern high-tech games, but their hardware can't support those games anyway. If they are only surfing the web and running basic tasks, they wouldn't benefit from an upgrade to 10 anyway.