I use vagisil which is slightly acidic. You want something that's a very mild acid, and many soaps are alkaline, which is bad. The vagina is naturally acidic, which prevents the growth of fungus (yeast/thrush) and the wrong kind of bacteria
There are various vaginal suppository gels that you can use overnight to keep the ph correct (florafem, balance activ), and you can also use dilute vinegar (apple cider, never balsamic)