Increasing muscle mass is very possible through a vegetarian diet. However, it is far more difficult to obtain muscle mass as opposed to eating meats. The complexity lies within creating complete proteins which entails eating a variety of different incomplete proteins to create allow for obtain all essential amino acids.
Ultimately for performance – there have been studies that show it is far difficult to keep up with a meat eater.
Fuel the Movement,
If increase in muscle mass is desired, a hypertrophy oriented stregnth training routine needs to be supported by adequate nutrition.
It is a fallacy to believe that a vegetarian diet is inherently deficient in protein sources. There are plenty of protein options, particularly when the person on a vegetarian diet includes dairy and egg products in their chosen meal plan.
A vegan diet (no animal products at all) requires greater case and planning but there are also plenty of options.