It depends in what context.
I'm a salesman, and I sell. If I was "vending" a product, it'll sound a little funny. You don't really say "I will vend you this pen for $5."
Now, maybe in the context of using a vending machine "vending" or "vend" the item. This is probably the only time you'll use the word.
In my opinion, I would say "sell" is NOT the same as "vend". Ask the guy who sells for a living. :-)