If things are the same as when vampires first came out (Shortly before I quit playing awhile back,) the vampire, without food, won't die but will slow to a crawl. He will only eat sleeping dwarves, I think, as well, though they might not have to sleep, but I am betting on them having to at least be living.
The army is not really effective because of the consumption of other dwarves and their sluggishness without that food source. Personally, I'd just feed the workshop vampire children. Confine him to a burrow and close off the place, but keep a child in there with a designated bed for when he is hungry. Obviously this makes it more complex, because you need stores of food and water for the vampire's food.
If you have an excess of children it is a great way to get rid of them though, I am sure.
Vampires aren't really very efficient, especially since you can't tell them who to eat. They might decide your legendary weaponsmith looks the tastiest at any given meal, for example.
I remember someone posting about the queen of their civilization moving into the fortress, this pallid bald headed noble looking female dwarf who was a vampire. That's probably the only time I wouldn't kill them off, the queen of your civ can eat whoever she damn well pleases.