Considering my heritage, I could've welcomed this addition to Bad Company 2, but I've held out on getting it for the longest time. Now that I've played it, my decision was justified.
It's not that Vietnam is particularly bad, it's just awkwardly skewed in a few different places. The maps are suspiciously flat, which reduces the role of Recons a great deal (my de facto class). Certain maps seem to favor one team over the other, leading to an imbalanced game. The terrain just isn't very interesting to navigate and there aren't enough guns. The limited weapon selection reduces encounter variety and makes everything repetitious.
Pass over this expansion, it's not worth it.