Does vinegar help cleanse the vagina if applied tropically and washed off?

Sure, but: Vinegar would somewhat cleanse the vagina and some women douche with half strength vinegar. But a mild soap and plain water outside the vagina and plain water inside the vagina would also be adequate for that purpose - without douching. Few women have any need to douche, and many women who douche end up having vaginal discharge due to alteration of the vaginal flora because of the douching.