a vegan diet simply means no animal byproducts
this still means you can eat nuts, seeds, pasta's, breads, oils, etc. those items have a lot of calories so don't think just cause you do a vegan diet you will automatically cut down.
having said that yes vegans are skinny for a reason. as far as will you lose muscle eating a vegan diet the answer is not necessarily. again it depends. if you are using var and working out i do not see you losing muscle mass
i did a 10 day water fast where i ate NOTHING. 0 calories a day, does it look like in my avatar i lose muscle? we need to get away from these bro science myths that you need meat to build muscle, yes it is easier but it isn't necessary. i applaud anyone who eats discipline, it isn't easy to be a vegan