I'll begin by directing you to vegan bodybuilder Robert Cheeke, who specializes in teaching people (especially men) how to build muscle on the vegan diet. Though he had a lean build as a teenager, he was able to find ways to bulk up by eating plants and working out - he has never taken steroids. Robert recommends the following protein sources: hemp, pea, rice, tempeh, tofu, quinoa, and spinach, and suggests consuming 1-2 grams per pound of bodyweight. Robert also recommends keeping your total caloric intake high for energy and to recover from exercise/training, and to eat frequently. 

Outside of being friends with Robert, I don't have much knowledge on maintaining the vegetarian diet as a male, so I reached out to my Facebook community for advice and this is what they advise:

• Eating whole plant foods, one should neither be underweight nor overweight. To maintain muscles you have to lift something heavy.
• No need for extra protein or fats. Fruitarian is good enough. Lots of water rich ripe sweet fruit + greens & occatioanlly a hand full of raw nuts = Ripped! Plenty of aminos in fruits & veggies to grow muscle. Protien has to be broken down into aminos to be used anyway. By past a step. Go direct. Strengthen digestion & the rest of the body!
• Workouts and muscles thrive off of lots of greens. If I haven't had kale my workouts and recovery times suffer. Hey, if it works for a gorilla....
• Do load bearing exercise and eat foods with enough calories and amino acids (or proteins). Simple!
• I use the "Kind Diet" approach, where I base most meals around whole grains and add protein (beans, etc.) and some type greens (kale, collards..). I occasionally will drink some Raw Vegan Protein from Sun Warrior also. I do yoga and distance trail running. I do not have any problem maintaining weight/muscle and I would actually like to lose a few pounds
• As a male vegan (18 years, vegan that is), who happens to be a cyclist, I'm constantly trying to figure out how to get rid of that extra 5 to 10 pounds. Maintain body weight?? No thanks! :)
• I don't have any issue maintaining weight. It's keeping it off that's the issue!! Damn you sweet tooth!! My local supermarket sells chocolate flavoured soy protein, which I make up with rice milk. Big batches of food made with TVP and scrambled tofu!! And tons of nuts & seeds...