How many incidents in which the subject's weapon was a vehicle were investigated by a female officer?
subject's weapon was a vehicle refers to subject_weapon = 'Vehicle'; female refers to gender = 'F'
SELECT COUNT(T1.case_number) FROM incidents AS T1 INNER JOIN officers AS T2 ON T1.case_number = T2.case_number WHERE T1.subject_weapon = 'Vehicle' AND T2.gender = 'F'