The origin of the Vietnamese strain of H5N1 remains unknown. The viruses in Thailand and Viet Nam form a separate “clade” within the Z genoytype. This clade persisted in Vietnam throughout 2003 to 2005. Genetically it remained remarkably homogenous through 2004 although some changes are now being seen in 2005 – some through mutation and others probably through newly introduced virus.
Yes there is illegal movement of traffic of poultry from China into Viet Nam but this depends entirely on market conditiosn. With depressed markets in Viet Nam at present it is unlikely to be very lucrative. Nevertheless it is a potential source of new strains of virus.
Z genotype H5N1 viruses first appeared in domestic ducks in 2001 and seem to have survived and thrived in domestic ducks since then. There is still much we don’t understand about why certain ducks die when exposed to these viruses and others don’t. However these viruses retain their high virulence when they are introduced to chickens.