Got my 3rd vampire, checked on legends viewer, the only one that matches (same gender, same deities relations) is one of the dwarves I initially though will never be emigrating to my fort, she was settled in a human town!! I even checked the town and it belongs to a human civ that is and has always been ruled by human kings...
I guess this throws out the window, that they vampires need to be historical figures that belong to a dwarven civ, it seems they can belong to any civ (or at least be settled in any civ's sites)
I'm assuming that when a creature settles in a civ's site it becomes part of that civ, I'm checking the legends for the last settlement entry and so far my 3 vampires:
queen vampire - last settlement in my parent dwarven civ's capital (duh)
clothier vampire - last settlement in another dwarven civ's mountain halls
mechanic vampire - last settlement in a human civ's town
I also got the visit of the human civ's lawgiver, a frogman vampire that had 100 something inventory pages of jewellery... interestingly around half the jewellery was owned by him and the other half wasn't, so I was able to dump-steal, like 50 inventory pages of human, dwarf and elf bone, hair and tooth jewellery...
It took over a season (and I have over 200 dwarves) to steal from him, while he waited for my stationed baroness to grant him an audience...
In legends he apparently struck down the human law giver and took his position.