Not necessarily. In the case of your dorf getting killed, that's, well, tough luck. I could imagine the vampire hunters don't necessarily killing your dwarves in the middle of a siege unless they transform or reveal and pose a immediate threat. Even if no, it would not be very different from the current justice system were a hammerer or the sheriff can kill precious members of your fort for crimes far less dangerous as sucking blood from others, and far more stupid as not making gold beds.
Going into a full vampire fort would be either a terrible thing you can fight against with the help of local or visitor vampire hunters (otherwise called by you, attracted by rumors or following pursuit of a vampire/monster) in which case you could consider the game over once everyone is a monster or it could be a conscious decision, in which you don't call for help and try to "dispose" of any vampire/witch/monster hunter that comes to help the living, and once you succeed on converting the whole fortress you can consider yourself done.
The game could even have a announcement where you could choose to either keep playing with the now undead/cursed fort with all it's perks and challenges or simply accept this as the end of the fortress and start a new one.