Place your berries in a blender and add the sugar and lemon juice. Blend until nice and smooth. Place a fine-mesh strainer over a large bowl and strain the blended berries, pressing on the solids with a spoon or spatula to get all the juices into the bowl. You only want the juice, not the seeds, so throw the solids away. Reserve about 1 cup of the berry puree in a small bowl, the rest will be for the topping. Cover and pop in the fridge until ready to assemble your parfaits.
Whip the cream and vanilla in a large bowl until stiff peaks form. Gently fold the reserved berry puree into the whipped cream. Fill a pastry bag with this berry cream and pipe into 6 tall sundae cups. (If you're not serving right away, you can keep the cream in the pastry bag in the fridge.) Layer the parfait glasses with whole berries, more berry cream and drizzle at the end with the reserved berry puree. Garnish with a few more whole berries and some mint.