If the old desktop is running well with Vista for what you use it for, I would leave well enough alone. You may run into hardware limitations. (Do not believe anything about how much minimal RAM you can get by with). You may have software you run on your vista that will need to be upgraded or replaced if you upgrade to 10.
At some point in six months to a year, if you see a machine running 10 and you have to have it, then bite the bullet and get a current version Surface Pro and dump both the laptop and Desktop, or keep one as a backup.