Old (Spanish contact period) maps, including those of the City of Vigan, can be found at the National Archives of the Philippines, which is also in Manila. (Please search the institution's contact details through their website, and the location through Google Map.) A few of these Vigan and Ilocos maps were included in the exhibition catalogue book "Conscripcion: Imagining and Inscribing the Ilocano World," which was published by the National Archives late last year. You may therefore try writing the National Archives, but I wish to remind you beforehand that such reference materials are protected by copyright, which means that access and use of these are bound by rules. I suggest that you consider this when you write the letter.