Yes vinegar does help to kill drain flies but you should also pour some boiling hot water down the drain as well to help kill the drain flies faster and kill off any drain fly eggs and to help flush the drain flies and there eggs down the drain.
To get rid of drain flies you should use 1/2 cup of salt, 1/2 cup of baking soda and 1 cup of vinegar and pour it down the drain.
Allow that mixture to sit overnight and then in the morning boil a pot of water and pour the boiling water down the drain to help flush the mixture and drain flies out of the drain.
You should also pour a pot of boiling water down the drain at least once per week which will also help kill any drain flies and drain fly eggs in the drain.