Create a Python program to reformat a JSON string, preserving indentation and enforcing the order in which members appear.
json_string = '{"fruit": ["apple", "banana", "cherry"], "vegetable": ["carrot", "potato", "onion"]}'
import json
 
def reformat_json(json_string):
    return json.dumps(json.loads(json_string), indent=4, sort_keys=True)