As pooya says above, you can't get the coins back if they're confirmed - that's part of the Bitcoin appeal, irreversible transactions. There's another possibility here: the vendor could be trying his luck. If you got the address from the vendor's site/page (you should prove where you got it, a screenshot perhaps?) and if your device isn't compromised (you scan and find nothing), you could also try look up the address, perhaps the vendor is already linked to it or linked to any of the addresses that the "spent" coins went to.

A bit tricky to do business with Bitcoin for newcomers but you probably know by now to confirm ownership of the address if doing direct txs with vendors. Which site did you do this on?

no, unfortunately for you bitcoin transactions are not reversible if they are confirmed and your transaction (based on what you said) is already confirmed.

your only possible option is to figure out where you got that address which you sent to. if you can find out the owner then you may ask him to send it back. otherwise if you copied a wrong address or for example if you have a clipboard-hijacker malware on your computer which changes copied bitcoin addresses, then you are out of luck.