Mix 1 cup of the water, vanilla and sugar together in a small pot and bring to a simmer. Remove from heat and let cool slightly, then add gelatin and whisk until fully dissolved. Stir in remaining 1 cup wine and set aside.
Carefully arrange berries at the bottom of a 5-cup nonstick loaf pan. (Remember that the bottom of the pan will be the top of the terrine once it is set and inverted for serving.)
Continue arranging berries, stopping about half an inch before you reach the rim of the pan.
Pour three-quarters of the gelatin-water mixture over berries. (If the berries are fully covered, they will float up from the bottom of the pan.) Chill remaining gelatin-water mixture in a bowl.
Chill terrine for 2 to 3 hours to set. Reheat remaining gelatin-water mixture just until pourable, then pour it over the berries to cover as much as possible. Chill again until fully set, 4 to 6 hours.
To unmold terrine, dip bottom of loaf pan into warm water for a few seconds, just until the gelatin starts softening on the sides.
Carefully run a knife around the edges of the gelatin to help loosen it. Place a serving plate on top of the loaf pan and in one swift motion, flip the mold over to invert it onto the serving plate.
Carefully lift off pan. To cut slices of terrine, dip a long knife in hot water for a few seconds. Dry the knife and while still warm, slice the terrine. Garnish with mint leaves and serve immediately.