You may think that buying swimsuit is hard because of the style, the offer, the size and so on. Here are 2 shops where you can go:
Bold Swimsuit: The shop is located on 433/8 Hai Ba Trung in district 3 not far from the pink church. They have really nice swimsuits for both small and big size and a large choice of one piece and two pieces ones. They sell everything separately meaning you can match different pieces and colors together. Count around 250k – 300k for the top, 230k to 280k for the bottom. You can check the price on their website. On their Facebook, they give you more precision on the burst and butt size. You can order online via the website, Facebook or Instagram, the delivery takes few hours and costs 20k in D1 & D3, 30k for other areas. Items can be returned/exchanged if needed within 4days.
Cora Swim: They sell their items at Kokois in district 2. It is mainly one piece swimsuits and price is around 1,500,000 VND.
Other alternative, you can check the Instagram accounts of the followings but you may need to have someone who can assist you in vietnamese: