After looking at this on FIFA official site, I have found some interesting points about Vietnam results against some regional and continental opponents:
Vietnam vs Singapore: The last time Singapore defeated Vietnam was in the final of Tiger Cup in 1998. It means Vietnam has been unbeaten against Singapore for 12 years.
Vietnam vs Thailand: Thailand is the opponent that Vietnam has met the most (18 times) since 1973 and Vietnam could get only 3 wins.
Vietnam vs Indonesia: Vietnam never considers Indonesia its biggest threat in any competition but the truth is Vietnam has not beaten Indonesia since 1999.
Vietnam vs Malaysia: Vietnam has never played against Malaysia in Malaysia since 1991. 6 of their 8 games were held in Thailand and Indonesia.
Vietnam vs Myanmar: The first time (since 1991) Myanmar won against Vietnam was in 2008. Vietnam won the rest.
Vietnam vs Laos: The first game 2 teams met ended as a draw in 1996. Since then Vietnam has won all 7 games against Laos. Laos has not scored against Vietnam since 1998.
Vietnam vs Cambodia: Vietnam has won all 5 games against Cambodia since 1995, scoring averagely 6.2 goals per match, the highest rate.
Vietnam vs Philippines: The last time 2 teams met was in 2002.
Vietnam has never met Brunei as well as Timor Leste.
Vietnam vs China: Vietnam has lost all of their 4 games against China, scoring 2 goals and conceding 15 goals.
Vietnam vs Japan: The first time was in 1973 and the second time was in 2007.
Vietnam vs South Korea: One win for Vietnam in 2003. Vietnam has played against South Korea 6 times from 1996-2004. More than most other teams.
Vietnam vs Turkmenistan: 5 times. Vietnam's first win was in 2009. The rest are defeats.
Vietnam vs Sri Lanka: 3 games in 5 days in 2002.
Vietnam vs Iran: Vietnam has never met this Asian powerhouse.
Vietnam vs UAE: 3 games from July to October in 2007. One win and two defeats for Vietnam.