This is why I love YT. I learn something new all the time. I have never experienced anything like that with any of my females. It does sound like a very unusual situation. The vets advise sounds logical. My main concern would be her getting a urinary tract infection. Here is a good article that pretty much explains what is going on.
In's and Out's of Puppy Vulvas
Keep in mind that most females do experience their first heat around 7 to 10 months of age occasionally some females don't go into heat until after they are a year old. The longer you wait to get the problem fix then the greater the risk that she can get an infection. I pray your little girl will be fine.