The two nutrients vegetarians can have a problem obtaining are methionine, an amino acid not found in plant protein other than perhaps fresh water
algae such as spirulina, and you can't really consume a whole lot of that at a time. So getting complete protein is important. The other is B12, which is also quite high in spirulina but quite sparse in a vegetarian diet. This is one of the reasons most people who claim to be vegetarians consume dairy, definitely not a plant, and fish.