The Vitamin C is my own. There are no dosage instructions, although I can tell you what I do that works for me, though it has not been a cure... just a treatment.
Firstly, I'll tell you why I felt that Vitamin C might work that made me try this.

If you do some reading on the nature of BV, why it causes problems (the overgrowth of the anaerobic(bad) bacteria and coincidentally the undergrowth of the good flora - which is most notably hydrogen peroxide producing Lactobacillus)

Then, do some reading on the specific species of Lactobacillus that makes up the majority of flora in the healthy vagina, you begin to discover that the "Lacto" fights off the anaerobic (anaerobic means that the bacteria cannot survive in a high-oxygenated environment, most "bad" bacteria including cancerous tumors are made up of anaerobic cells and bacteria) anyway, the good stuff "Lacto" fights off the anaerobes by releasing Hydrogen Peroxide (H3O - which means 1 Hydrogen atom and 3 Oxygen atoms, if you know your chemistry, you know that water is H2O - water is stable, Hydrogen Peroxide is not - the extra Oxygen seeks to bind with something, it "oxidizes" what it binds with) Which is what you see when you pour Hydrogen Peroxide over an infected cut - the extra oxygen atom is "oxidizing" and neutralizing the anaerobic "infecting" bacteria.

I tried using Hydrogen Peroxide as a douche, and it works to immediately eliminate the odor, but the one problem science hasn't tackled is how to facilitate the regrowth of the "Lacto" - hence the vaginal flora never returns to normal. It is apparent by the studies that the vagina needs a very acidic environment. Vitamin C seems to promote the production of Hydrogen Peroxide at the same time creating an acidic environment.

Here are some links to some articles I've read
http://www.webmd.com/content/Article/111/110117.htm?pagenumber=1

http://www.garynull.com/Documents/Arthritis/Hydrogen_Peroxide_Therapy.htm

(This article contains the paragraph below that I cut and pasted here to highlight the portion I am referring to)...

[Before scoffing, keep in mind that one of the very first lines of defense against any and all microorganisms recognized as invaders by our immunological system are macrophages and leucocytes, one of which uses hydrogen peroxide to oxidize the foreigners; and that vitamin C is effective principally by its ability to promote hydrogen peroxide use against foreign invaders, including parasites, viruses, bacteria, yeast/fungus; and that all body tissues contain catalase and that hydrogen peroxide in the presence of catalase is reduced to oxygen and water. So, there is strong reason to believe that added hydrogen peroxide, used properly, may be both effective against certain organisms and safe.]