I'm a vegetarian, and I'm certainly not malnourished. You just have to make sure you eat enough of the meat substitutes, and you should still eat seafood regularly, even if you limit it to the literally brainless bivalves (clams, mussels, etc.). Vitamins help, too, as does fish oil if you don't want to eat seafood regularly.
Have you been diagnosed by a medical professional or have you just decided on your own that you are malnourished? If you are, in fact, malnourished it would indicate that you do not have good discipline when it comes to your diet. A vegetarian diet requires particular attention to getting a full complement of nutrients. Perhaps you should give yourself a bit more latitude (i.e., variety) in what you eat until you get healthier, then try the restrictions of a vegetarian lifestyle.