Is vegan healthy?
I like tomato's and lettuce on my burger, and I've never had a problem with it.
A lot of the animals I eat survive on grass, and other grain type foods. They seemed healthy enough for me to eat them.
Can a vegan build muscle?
Their have been bodybuilders who were vegans. To what exact extant I can't tell you. Being vegan is not something I have really ever entertained. I like meat!!
A lot of people talk up Soy as a wonder food, but I can find plenty of info bout Soy that says it is not all that great.
Here is a very good site about nutrition. It is large, and will require some of your time to get through all the topics that they present.
The Weston A. Price Foundation
I'll add this. Nobody should just stop eating protein, and take up a vegan lifestyle until they truely understand amino acids, and how your body works.
Living as a vegan requires a lot of education to eat properly.