Yes it is possible and no you don’t have to combine your proteins. Your body will do that for you. Increasing muscle mass, besides having the right strength routine, will involve eating a ton more food period. There are plenty of vegan and vegetarian sources of protein as well as bodybuilders and other athletes who are vegan or vegetarian. Stores like The Vitamin Shoppe and health food stores are pretty good about keeping protein supplements in stock that have no whey or casein. Once you start looking, you will find that there are more options than you can possibly imagine.
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.
Definitely vegetarian diet can work to increase the muscle mass. People adopt this diet for different reasons including heart health, ethical concerns and religious views. Some people adopt it to lose weight, but gaining weight with vegetarian diet can be challenging. You need to add certain foods that can increase body mass or muscle without any side effects. You can prepare healthy vegetarian diet chart based on the tutorial of health experts like Boris Wolfman – http://boriswolfman.co.il/%D7%90%D7%A8%D7%95%D7%97%D7%94-%D7%98%D7%91%D7… and add some important things like dairy products, starches, bananas, tropical fruits, soy products, nuts, seeds, protein shakes and many more in your daily routine.