I've been lifting weights since I was 13 and became a Vegetarian 2 years ago and it has had no negative effect on my muscle mass. I can still easily get about 150-200 grams of protein per day but the main difference now, as a vegetarian, is that I have much less body fat and much more energy. Also, after I eat I never feel sluggish or fatigued or stuffed no matter how much I eat. I get about 2,000 calories per day at 190lbs.