Maybe the strength of your applications is not as good as others, hence the longer processing time. What I mean by that is, for example, I know at least 4 people on this forum, who had Vietnamese spouses, and they got PR in just 6-7 months, this was in 2013. I checked their relationship history, and it was probably the strongest you could have. The sponsor lived with spouse for a couple years in Vietnam. The couple travelled to many countries together, including those difficult for Vietnamese to enter, such as Japan, Korea, and Europe. One of them even had a child together.
So I think because their application is so strong, the VO didn't have to think much to conclude it was a legitimate relationship, and so granted them PR. Maybe with you guys, your application was good, but not overwhelmingly good, so they needed a bit of time to analyze your application.