Why is "do" not used instead of "de"? de + o vinho = do vinho? I get confused in this area. Thanks!
Because it makes it specific: do vinho = of the wine. You can use it to specify:
Uma garrafa do vinho Bordeaux, for instance.
"de" is general. If you use "dO", you are talking about some specific wine, once that "o" is the masculine definite article. Eg: a bottle OF wine / a bottle of THE wine.