Making any change in your life, especially related to diet plan, is a process. Like you asked about a vegetarian diet, it will allow you to lead a healthy life and surely help you in increasing muscle mass too. The hectic lifestyle that we all lead today does not do much good and therefore we have to focus on what we eat. Eating green vegetable and fresh fruits can do much good not only to increase your muscle mass but it would also help you maintain your body weight.
I often visit Sheffa Foods for a list of some amazing health foods and products that I think are great if you want to lead a life that is healthy and disease-free.