Toss cranberries in lime olive oil and spread evenly on a baking sheet. Roast until cranberries swell and burst, about 5-7 minutes. Remove from oven and set aside to cool.
Combine vinegar and sugar in a small saucepan over medium heat and stir to combine. Simmer until the sugar dissolves, stirring occasionally. Remove syrup from the heat and set aside to cool.
Coat the bottom of each glass (rocks or martini) in 2 teaspoons of syrup and add 1-2 teaspoons of roasted cranberries.
Fill a shaker with ice, add gin and lime juice, shake to combine and pour ingredients into prepared glasses (strained or with ice).
Splash a little ginger beer on top of each cocktail and garnish with lime slices before serving.