Purée cider vinegar, oil, egg, salt, poultry seasoning, and pepper in a blender until smooth.

Pour marinade into a resealable plastic bag. Add chicken halves, coat with marinade, squeeze out excess air, and seal the bag. Marinate in the refrigerator for at least 4 hours or overnight.

Preheat an outdoor grill for medium-high heat and lightly oil the grate.

Remove chicken halves from the bag and transfer to a paper towel-lined baking sheet. Pat chicken dry with more paper towels. Reserve marinade.

Place chicken, skin-side down, onto the preheated grill over direct heat. Grill for 2 minutes, then turn each piece, brush with marinade, and move to indirect heat.

Continue to cook chicken, turning often and brushing with marinade, until nicely browned and meat is no longer pink in the center, about 45 minutes.