I prob wouldn't deworm a baby that young unless you really feel there's a reason to do so. Are the cows heavily infested with worms?...Or are your pastures terribly overstocked? If there's a high probability the calf has contracted worms already, then you prob need to tweak management of the entire herd. As for vaccinations, early immunity should be transferred from the mother in her colostrum, so if the cow was properly immunized and the calf got enough colostrum, then that shouldn't be the problem either. It kinda sounds like this calf may not be getting a lot to eat. How's her mother's milk production? Would the calf respond to being creep fed? If you have reason to suspect that treatment of some parasite or disease is necessary, then by all means, do it. I just don't like to throw solutions at a situation without trying to figure out what the problem is.