You don't have to substitute. There are no different "dire" consequences than if you ate a standard meat diet. If you eat Junk food you will still be susceptible to the diseases that all Americans who eat poorly are. Heart disease, diabetes, arthritis, etc.
Vegetarian does not mean healthy. The only think you have to do to make it healthy is to eat whole plants, grains, fruits, etc and lots of them. There is no need to substitute anything for meat.