Cut rind off watermelon and dice into chunks.

Puree watermelon chunks in a blender until smooth (about 30 seconds).

Place all but 1/2 cup of the puree in the refrigerator to chill.

In a small saucepan over medium-high heat, bring the 1/2 cup of watermelon puree and the syrup just to a boil, stirring constantly.

Remove from heat and stir well to blend.

Let mixture cool completely (about 20 minutes).

Add orange juice, orange zest, salt, and vanilla.

Add the cooled mixture to the remaining watermelon puree.

Place in refrigerator for about 15 minutes to ensure mixture is well-chilled.

Using an ice-cream maker, freeze sorbet according to manufacturer's instructions.

Transfer to a glass bowl or baking dish, cover tightly with plastic wrap or lid.

Store in freezer for up to 1 month.