Combine strawberries and mint into a glass or plastic bowl.
Add sugar to strawberries.
Cover and refrigerate for a minimum of 4 hours to 24 hours. The longer they sit, the more juice you will get.
After at least 4 hours, add vinegar and mix thoroughly. Adjust flavor here. If it is too vinegary, add some sugar. If it is too sugary, add some more vinegar. It comes down to your taste preference. I enjoy a little more vinegar solution.
Strain the strawberry mixture through a fine mesh strainer. (You can also use cheesecloth.)
Use immediately or store in the refrigerator until ready to use.
For the Cocktail
Combine all liquid ingredients into a shaker filled with ice.
Shake and strain into a chilled martini glass.
Garnish with a mint leaf and piece of lemon peel.
Notes
I use White Wine Vinegar or Champagne Vinegar because it is light in flavor and compliments most fruits. I also use a lot of seltzer water because it has little to no sodium whereas club soda is usually high in sodium.