A Vietnamese filter is a small coffee pot. It looks like a hat and sits upon the top of a coffee cup. Inside is a chamber for coffee and room for hot water. It is very basic and simple, and works!
A Vietnamese filter, coffee (optionally with chicory), and condensed milk. Condensed milk typically contains sugar and is heat processed with steam. Steamed milk has different physics from non-steamed milk and tastes better when used in coffee (ask any cappuccino drinker).
There is controversy here, I have read of anything from fine to coarse grind being used. The cans have what appears to be a French grind; I use a grind somewhere between French and drip. This does not fall through the holes in the filter and tastes great. If you are new to Vietnamese coffee, use a French grind since it is easier to get it right with a coarser grind.
The typical way to serve the coffee is with condensed milk (not evaporated milk). This adds a wonderful sweetness and mouth feel to the coffee. Additionally, the water temperature is very important, see notes in the French Press section at HowToBrewCoffee.com what else should I know? (next page)