I want to be able to annotate a 3d model with billboards, but I cant seem to figure out the transformations to make it work. In other words, I want a billboard that can be "tacked" to a vertex on a model, move as the model is moved, but always face the camera. Thanks.

Fetch the "up" and "right" from the view matrix, and use this to construct the billboard and thatll make it always face the camera.
Its not that hard, what dont you understand about it in more detail?