How can I expose a vampire?
Solution 1:
According to the wiki, Dwarf Therapist will show his original name, which means comparing the unit list in DF to the list in Dwarf Therapist and eliminating all dwarves that appear in both. The last name in each list will show the alias in DF and the real name in Dwarf Therapist.
Alternatively, you can make a copy of the save, enter Legends mode, and then look up a dwarf that was killed by your vampire. It should say "in the Year N, Dwarf X was drained of all blood by Vampire Y".