Find a Vista install disk and install it with a valid license key. If it originally came with Windows Vista you might still be able to download all necessary drivers* from the makers site.
But if I were you, I would go back to the shop where you bought it and return it if they can't get Windows 10 running on it.
Kees
*Note: Typo corrected by moderator edit.
I recently brought a laptop that I brought because it was advertised in shop window as windows 10.
When I first used it, Windows 7 was running. I kept getting a message saying that "Your PC is compatible to run Windows 10."
I downloaded it is not running.
I took my PC to a different computer shop to see if the could help. They informed me that it was only licensed to run Windows Vista.
Can anyone tell me if it is possible to downgrade to Vista home, and if so how ?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic