The agreement to sell six Kilos to Vietnam, was announced in mid December 2009. The Kilo 636 is not the world's most advanced conventional submarine (that title may go to late model HDW 214s) however the Kilo's up front price is relatively inexpensive. Vietnam's Kilos will not have AIP. Vietnam also benefits from strengthening its political, economic and strategic relationship with Russia.
Vietnam is courting closer relations with Russia, India and the US as a counterweight to the China threat (on land and sea) to Vietnam. The asymmetric power of Vietnam's submarines will provide limited defence of Vietnam from the risk of Chinese sea-based invasion.
Vietnam is also competing with China and other nearby countries for resources under the South China Sea. Submarines provide a cheaper alternative to expensive surface ships (like aircraft carriers) for Vietnam in that competition. Vietnam has no submarine tradition outside of owning two old midget Yugo Class submarines. It will take years of Russian training to make Vietnam's submarine flotilla efficient. Russia will regain some of the naval intelligence and military power it lost when Russia withdrew from Cam Ranh Bay.
Vietnam's Kilo Subs Steadily Being Built By Russia, July 7, 2013 http://gentleseas.blogspot.com.au/2013/07/vietnams-kilo-subs-steadily-being-built.html and
Vietnam's Evolving Nuclear (Energy) Program (with close Russian involvement), October 12, 2013 http://gentleseas.blogspot.com.au/2013/10/vietnam-signs-ambiguous-nuclear.html