A list of the best Vietnamese coffee brands only helps once you know what you are buying. Here is the short version.
Robusta share. Around 95 % of Vietnam's harvest is robusta, and the classic cup is built on it. Blends with some arabica soften the cut; pure robusta gives the strongest, most chocolatey result. If you want the full picture, read Robusta vs Arabica in Vietnam.
Roast. Traditional Vietnamese roasts are dark, sometimes with butter, sugar and flavourings (rang bơ). That is where the caramel and cocoa notes come from. A pack that says “light roast” is not what you want for a sua da.
Culi. If you see culi on a pack, that is peaberry: single small round beans. A common choice for a punchy cup.
Grind. For a phin, ground medium, or whole beans you grind yourself. Packs marked for the phin are usually ground right.