The Bodhisattva Vow is taken some time after one has taken the Refuge Vow. When one understand what a bodhisattva is and what it entail, one can take this vow. It should be done from a qualified teacher and does not have to be from one's root guru.

A bodhisattva is described in great detail in The Bodhisattva's Way of Life. There are many shorter teachings on bodhichitta or mind of enlightenment that one can study.But bt basically is that from this point on, one will behave as a bodhisattva and help all sentient beings until samsara will finally be emptied. Basically, this means one commits to helping others for this lifetime and many, many more lifetimes.