I had returned from Vietnam a few months ago. My partner (who is Vietnamese) and I visited a pearl farm on an island called Phu Quic. However, this is information my partner had told me, that island has a very bad reputation for scamming people. Although it is not as bad these days due to government crackdowns and regulations, you still need to be vigilant.
Back on topic, the pearls I saw are of ok to good quality depending on price. However, there is a catch, the quality to price is vastly not worth it from what I can get them locally in Australia, so this may differ depending on where you are from. I can pay cheaper, for even better quality at my local jeweller/dealer. For example, some pearls are of lesser quality than my Paspaley, yet double the price. We did not see any pearls that matched the quality of my Paspaley, and they thought such pearls couldn't exist or were fake.
I would suggest you be vigilant if you are to buy pearls there.
When I visited Vietnam I only saw Chinese freshwater pearls being sold at shops, and the sellers were almost all claiming the pearls were Vietnamese Akoya.