To make the spicy simple syrup, combine sugar, water, and sliced jalapeno in a small saucepan. Simmer for about 8 minutes, until it has thickened to a syrupy consistency. Remove from heat. You can store this in the fridge for several days.
To make the cocktails, run a slice of grapefruit along the rims of 2 glasses and dip into kosher salt. Fill with ice.
Combine grapefruit juice, vodka, and simple syrup to taste in a shaker filled with ice. Shake until chilled. Strain over the prepared glasses. Serve immediately with extra jalapeno slices as garnish, if desired.