if you are looking to build muscle the only way is to exercise and feed good food and feed the correct amount for your dogs activity level and size. if you are feeding your dog an apropriate food i personally don't see any reason to add vitamins, unless suggested by your vet. our dogs do get a scoop of plain yogurt every night with thier dinner, mostly to stave off thier notorious pit bull gas
that and they really like it. also i recomend talking to your vet about geting your dog on or off of puppy food.
our dogs go to the river to swim, get regular walks and get at the very least half an hour a day of good play in the back yard, like fetch or tug. while it isn't the best it keeps our couch warmers in good shape.
while i agree a pit bull is one of the worst gaurd dogs just because of thier people loving personalities, i will vouch that when some jerk thinks about breaking into my house and sees Boomers big old head they definitely think twice.
treat your dog as part of your family and they will come to your aide when it is needed.