help with documents

Hi there I am was wondering if anyone in Vietnam could help steer me in the right direction. My mum fled Haiphong to Hong Kong back in the late 70's with only our clothes on our backs, I am currently trying to locate my original birth certificate, my mum has found a copy of my olders brothers birth cert which should have the exact info as mine. I have been advised to write a letter to try and locate my details inroder for a duplicate to be made. I do not know who to write to for starters, the VN embassy in London said it should be The Department of Justice 56-58-60 Tran Phu St., Ba Dinh Dist., Hanoi, the USA visa travel website indicates Non-Residents can only get relatives resident in Vietnam to request extracts of documents for their overseas relatives. Does anyone know what I should do. I have asked around and my friend said I need a house book. without a house book I can get one for 14 million VN Dollar... which is crazy.