There are two different cuts of corned beef regularly sold at grocery stores, the flat cut and the point cut. While you can use either one, I strongly recommend the flat cut for sous vide cooking. The point cut is a fattier cut with more connective tissue and it definitely can be more flavorful due to the increased fat content, but it also has more connective tissues which can result in a tougher texture. I did not test the point cut using sous vide, but you can absolutely use it. You may find that you have better success if you increase the sous vide temperature to around 180℉/80℃ because the higher heat will break down the connective tissues a little quicker.