To take this vow means that once you are a step from enlightenment and does not have to come back to samsara, you voluntarily forgo enlightenment so that you can come back to samsara to help others. This vow is not taken lightly. For most Mahayana Buddhists, this does not mean just this life time, but for all subsequent life times. Of course if you do not believe in "rebirth/reincarnation," the gravity of this vow is probably not as serious.