It can go either way. It all depends on the pressure gradient, and it flows from high to low. Gravity matters, as does the pressure within each organ at either end of the fistula. If the fistula communicates with the skin, that is where the pressure is the lowest, so it always leaks out of the skin, instead of the other way. I doubt one is better than another. I guess the worst is if your bladder or urethra connects to your pouch, as that leads to chronic urinary tract infections. There is no urinary connection with a rectovaginal fistula.
The vagina is in front of the pouch, but behind the bladder. Basically, the bladder is closest to your lower abdomen and the pouch lies on the part of the spine in the pelvis, the sacrum. The vagina is in between.
Where in the vagina the fistula is makes a difference too, as they can be near the outlet, or as high up as near the cervix. The higher it is, the more difficult it is to treat.
Fistulas can also make crazy, tortuous paths, that twist and turn, and that is part of the difficulty in finding them. They can connect to the skin, bladder, vagina, scrotum, and even some other place in the intestines.
Some people have fistulas with no symptoms, but eventually symptoms usually appear. Or, what is more likely, is that symptoms attributed to something else eventually are found to be caused by a fistula.
Jan
