As you may or may not know, building muscle is about a couple of things:
1. Providing sufficient training stimulus
2. Providing enough carbohydrates for optimal performance
3. Providing enough protein and fats for optimal recovery
With the vegan diet however, point 3 on this list may be at a downfall.
This is due to the fact that most plant sources of protein do not have an optimal protein content, nor are they highly biologically available.
Most plant-based athletes have to mix a variety of grains, beans and legumes to derive the sufficient amounts of protein, and even then, that protein just doesn't have the same effect as animal protein.
Nevertheless, that doesn't mean a plant-based trainee cannot gain muscle.
It just means that the gains will not be made at the same rate as they would be with the highest bio-available foods (animal foods).