"glycogen supercompensation" dieting definitely works. You eat zero carbs for about four days, going ketogenic. (Eat plenty of saturated fat instead; lots of butter.) Then you lift extra hard. Then for the rest of the day you eat a ton of carbs: loads of potatoes, whatever.

I don't think it's possible to put on a lot of muscle mass without either eggs or whey protein. If your vegetarian rules keep you away from even those protein sources, you may have to rethink your priorities.