Embryo freezing is actually the method used before the vitrification technique was discovered. It was a slow, progressive process that could last up to 2-3 hours. The survival rates were not too high due to the damage caused to the embryos by ice crystals. Although it allowed day 2-3 embryos to be frozen, blastocysts were unable to survive.
Embryo vitrification, conversely, is a ultra rapid procedure that can last just a few minutes, and most importantly, it allows for blastocysts to be cryopreserved as well. This technique is not harmful for the embryos, since it prevents the formation of ice crystals.
You may also enjoy some further information reading this: Cryopreservation & Vitrification of Embryos, Sperm & Eggs.
Suggested for you: What Is Embryo Vitrification? – Advantages over Slow Freezing.