A "victory" is a win in a war, a battle, a contest, or game, against an enemy or an opponent. The word "victory" is used when the speaker thinks the event is important. Examples of use: "The Seattle Seahawks football team won a 37-18 victory over the San Francisco 49ers at CenturyLink Field." "Napoleon won his greatest victory in the Battle of Austerlitz." You would not use the word "victory" in the following examples: "A Framingham man has won his second $1 million prize from the Massachusetts State Lottery in a 10-month span." It's not a "victory" because the lottery is not his enemy or his opponent. "This is a picture of Carrie Fisher seven years before she won fame in 'Star Wars.'" It is not a "victory" because, again, there is no enemy or opponent.

I'm not sure; using both "win" and "victory" seems redundant, as saying you won already implies that you were victorious. It's only redundant if you're not saying it for emphasis.

Since "win" and "victory" are synonymous, the term "win a victory" is just a more complex version of "winning".