It seems like, if we are all being completely honest, you won't be able to maintain as much muscle compared with a diet that includes animal protein...and the muscle you do have takes constant work to maintain. Does that sound right?
I don't aim to increase my muscle mass (just my strength) so I might not be the right person to answer. But surely, maintaining a lot of muscle mass takes constant work regardless of whether you're vegan or eat loads of meat? Non-vegans have to be sure to eat a lot and work hard in the gym in order to gain and maintain muscle mass, and it'll be the same for a vegan.
I do think that if you want to have a lot of muscle mass, it's going to be hard to do that while eating really cleanly. It's easier to get lots of calories and protein if you are less discerning about where those calories come from and are happy to have protein shakes and veggie burgers and stuff like that. But bulking up with lots of muscle isn't really a healthy enterprise to start with.
If you want to have a really clean, whole foods diet, bulking will probably be harder, but you'll probably be healthier.