Communication is a very important factor in a happy marriage. The couple must be able to understand each other and thus language is an important tool of communication. if anyone can say that it doesn't matter if the couple understand each other, as long as there's sex, then this guy surely doesn't know what is marriage. marriage is more than sex and it takes both hands to clap. For example, the wife gets frustrated because she cannot understand you most of the time, then the chances of the husband getting sex is really low.
For bros who are hoping to have vietnamese spouses, its always good to learn their language but it will be even better if the spouses learn english or mandarin. It will depend on whether u are going to stay in Vietnam or she going to stay in Singapore. If she is coming to singapore, then let her learn english or mandarin, it will do both parties lots of good