Tough one. Everybody likes to get something for free, specially when it is someone else making a mistake. Not getting charged for something is like hitting the jackpot. Sadly it has never happened to me. One time it almost did, but a few months after I made the purchase, the charge showed up. That wasn't very nice.

However, noone likes to get burned about a second charge from the same place. I have had this happen on my debit account before, then mysteriously, with no involvement from me, the extra charges were reversed. Nice.

End result: call them and tell them about both. If the charge never came through from the vendor, then there is nothing Visa can do. They can take care of the extra charge. By calling, you feel good about doing the right thing.