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 are two schools of thought here. My understanding is that the authentic answer is coffee with chicory. I have very successfully used canned (!) coffee from Cafe Du Monde or French Market, both available in cans in supermarkets, both are coffee and chicory. (Chicory is a caffeine-free herb whose roots are dried, ground, and roasted and is used to flavor coffee.) I also have had great luck with fresh grinding my own beans without chicory. I often use Peets House Blend beans that I grind between drip and French. My personal preference is sans chicory ... but then it's not Vietnamese coffee. Poetic license, I guess.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?