How do you get a bacterial infection of the vagina?

A number of ways: Most common stds are bacterial, so sexual transmission is one way; alternatively, you can have fecal contamination if fecal matter made its way to the vaginal vault. Anything that flushes external material into the vagina, such as douching, could lead to a bacterial infection as well.

Bacterial vaginitis: This may be caused by a variety of physiologic changes including the acid/base balance in the vagina, the various types of "normal" bacterial flora present. This can be sexually transmitted, in the sense that organisms not previously present are introduced, but this is not a common etiology except with gonorrhea.