Vows and Oaths are two different ways to invoke Divine Magic, they could be done via sacrifice, artifice, runes, or simple prayer.
Warning: Do not try to try flippancy with Vows and Oaths, these magicks are sacred to the Gods or Spirits you are trying to invoke, and you will no doubt not get any magic WHATSOEVER if your flippant and disrespectful.
Vows and Oaths are also good for getting a little extra magic for certain high level magicks, and it is at the God's or Spirit's judgement on how much power you the Vow or Oath taker will get.