Can I use the same bitcoin address to receive more than one payment? I used an old address I once used to receive a payment; can I still receive funds in that address even though the address on my bitcoin wallet has changed? The vendor claimed he made the payment but I can't find it in my wallet. What do I do?
An old address can be reused, but its usually recommended you don't do so for privacy reasons. 

Even if you wallet is displaying a new address, it should still show a balance that includes any recent payments made to older addresses. 

To be sure, check the receiving address on a block explorer website, and if the vendor is telling the truth, you should see the recent payment in addition to any past transactions. At that point,  try re-syncing your wallet if the payment isn't displaying properly.