Put the raspberries into a large, shallow dish and mash with a fork or potato masher. Transfer the fruit and its juice to a 1-quart Mason jar and cover with the vodka. Seal the jar, shake to mix, and set it aside in a cool, dark place for 6 weeks. Strain the contents of the jar through a fine sieve into a 4-cup glass measuring cup. Discard the fruit; rinse the sieve and the Mason jar thoroughly. Fit the sieve with a damp coffee filter and strain the flavored vodka back into the jar. Combine the sugar and water in a small saucepan. Bring to a boil over medium heat, stirring until the sugar dissolves. Reduce the heat to low and simmer for 5 minutes, or until a clear syrup forms. Remove the pan from the heat and cool to room temperature. Add the cooled syrup and the glycerin to the liqueur. Reseal the jar and shake gently to blend. Using a funnel, divide the liqueur between two 10-oz decanters. The Raspberry Vodka Cordial should be ready to drink in 2 or 3 days and has a shelf life of 2 to 3 months.