I just came back from Vietnam last week. My advice for buying pearls in Vietnam would be "DON'T." There are pearl shops everywhere, especially in Halong Bay. The sellers are all ready with presentations about pearl farming and stories about the industry, but I did not see a single strand of genuine Vietnamese akoya at any of the shops. They all carried commercial grade Chinese freshwater pearls at massive markups. Every time I told them that they were from China the reaction was incredible.
Oh, and one seller was even selling shell pearls claiming that they were coral.