How do probiotics taken orally actually reach the vagina?

It could also be a result of the way Lactobacillus alter immune function and the internal environment of the human body. They might encourage the indigenous population of Lactobacillus in the vagina to flourish. Studies so far have concluded that oral administration of probiotics positively influences the vaginal microbiome, but they haven't assessed all the possible mechanisms for this.

Ok how do they naturally gravitate to where they are needed. You have not answered anything here. First at all probiotic are bacteria. Bacteria can not enter the blood stream otherwise you would have bacteremia. So As you ingest the probiotic it has to come out through only one way out of your body. It can not enter your urine as it would breach the blood to enter the kidneys and to be filtered out. So there is only one way out and that is through stool. So small amount of healthy bacteria such as lactobacillus remains at the perianal area where its quit moist. The daily activity eventually gets it to the vagina. Where it can reproduce in larger quantities. It has to come out in the feces. The reason why e.coli does not live in the vagina its because of competition with lactobacillus and other beneficial bacteria. Its natural selection on microscopic level.