Put water, sugar, and cranberries in a medium saucepan and bring to a quick gentle boil.

Once boiling, lower the heat and let it simmer for about 10 minutes, until cranberries have burst (I also smoosh the cranberries with a spoon once they've burst).  Around the 5-minute mark, add the cinnamon stick.

Remove from heat and stir in the splash of vanilla and pinch of salt.

Strain the mixture into a large measuring cup, pressing down on the cranberries with a rubber spatula to extract as much juice as possible.  I recommend doing this within the first 10-15 minutes, or else the mixture will start to turn into more of a jam (thickening up quite a bit).

Once it's cooled a bit, pour into a mason jar or simple syrup bottles and place in fridge to cool completely before using.  Note, the syrup will continue to thicken as it cools. 

This should keep for a couple weeks in the fridge before starting to go bad.