I don't really like the word fanjo and have never used it. But I do think there is a lack of a sensible usable slang word for female genitalia.
I know all the arguments for using 'proper' names, but I really cannot imagine using words like penis, anus, vulva etc in conversation. Surely nobody refers to their buttocks rather than their bum or bottom in normal conversation? All these words seem strictly medical to me rather than something I would use in conversation.
So in our house we use willy and girly bits. On the basis that when the kids are old enough I can explain the different 'bits' girls have down there in more detail, and that it will be understood by anyone that needs to (eg infant school teacher "please miss, my twinkle hurts" ER????)
Nobody really seems to care that the vast majority of the population refer to a willy or something ruder rather than a penis, so why should people refer to a vulva?
Incidentally, in a sexual context, I don't think I have ever used any words to describe any bits, so I am only really talking about talking to children. If I was talking to a doctor I would use the proper names, and I am unlikely to talk to anybody else about my bits!