In a large bowl, whisk the oil, lemon juice, syrup, salt, vanilla seeds, vanilla bean and bourbon. Place the peaches in the marinade. Toss to coat. Allow to sit covered at room temperature for 30 minutes.
Preheat an outdoor gas or charcoal grill until very hot, or put a grill pan over medium-high heat. Lower the heat to medium. Using tongs, remove the peaches from the marinade, reserving the liquid. Place them on the grill, cut-side down. Allow the peaches to sit on the grill until the grill marks the peaches, about 5 minutes. Lower the heat to low, close the lid and cook covered for 15 to 18 minutes. They should be caramelized but not mushy.
Using tongs, remove the peaches from the heat and place them back in the marinade. Allow them to cool to room temperature. The peaches can keep for up to a day covered. Don't refrigerate these babies!