Add vinegar and berries to a medium saucepan and bring to a boil. Reduce heat and simmer until liquid has been reduced to about 1 1/2 cups. Strain berries out through a strainer and return liquid to pan. Make sure all berry seeds have been removed. Bring liquid to a boil and stir in cornstarch mixture, a little at a time, until liquid thickens. Allow to cool and transfer to a squeeze bottle. Syrup will thicken as it cools.