Yea, since it's not actually free. You are transferring your old OS license to it.

I got win 10 pro on one of my laptops on the weekend. Oh man, what a cluster mess of an operating system. Already loaded with shovel ware and just, well, a gigantic mess.

I'd happily install vista over the top of it. BTW, there was nothing wrong with vista. It's just as slow as win7 was. Win8.1 ftw.