Yes sadly not, I have a crappy originally Vista Laptop I stuck Linux on simply because thats all it was worth doing to, if I could have tried Windows 10 on it I might have tested and liked it enough to risk doing on my main desktop but no way will I do that otherwise.
Its as if Microsoft realized Vista users were prob the unhappiest of all Windows users and the only ones who would leap at the chance of an upgrade so they are cruelly trying to make them pay.
You are correct it is not possible to get Windows 10 for free if you are using Windows XP or Vista.
It was possible previously by installing the Windows 10 Preview edition and then upgrading that, but now you will need to pay. Although time is running out now, it might have been worth buying a cheap Windows 7 upgrade (around £30 in the UK) and then grabbed the free Windows 10 upgrade.