In a small pan over medium heat, add the sugar and 2 cups water and stir to dissolve. Bring to a boil, remove from the heat, and cool.
In the saucepan with the cooled sugar syrup, whisk in the maraschino cherry syrup, half-and-half, and vanilla. Pour into an 8 by 8-inch cake pan and freeze for at least 2 hours or overnight.
Fill 4 tall glasses 3/4 full with cola. Add 2 scoops of the frozen cherry mixture to each glass and top with a cherry. Add 1-ounce vanilla vodka, if using, to each glass.
Scrape the remaining frozen cherry mixture into a resealable freezer container and keep in the freezer for up to 1 week.