A while back while working in the oil industry I had an issue with my own and some of my men's passports being held while we were working in Vietnam. After my requests and then demands for return of our passports was refused I called the UK Foreign Office (Tel:020270081500). These guys were great and on the phone to the Vietnamese authorities immediately and I had the passports returned the next morning. I always keep this telephone number with me now when working overseas. The other advice on getting a second passport is possible and is a good idea.