Some say it's incorrect usage.
Some say it's a 'social' or 'conversational' usage, and the people using 'vagina' like that also know the actual terms for the parts.
Has anyone looked into this phenomenon at all? Has anyone bothered to do any sort of scientific poll to see if women who use 'vagina' in a non-traditional way also know what their parts are really called?
When does a popular misuse of a word become a correct usage?