If you live in a city with a sizeable Asian community there's a reasonable chance you can get actual Vietnamese coffee these days, usually cheaper than any of the New Orleans chickory blends. (Trung Nguyen, the leading Viet brand, seems to've greatly expanded their availability in the US in the past decade or so.) And a Vietnamese coffee filter is usually only 3 or 4 bucks. Cafe du Monde is often available at regular American grocery stores, but at a pretty steep markup for what it is.

The key is that Vietnamese coffee generally isn't Arabica coffee, the smooth variety most common in American coffee these days; it simply doesn't grow well in Vietnam. It's mostly Robusta, which is rather bitter, along with other 'inferior' varieties. This, combined with the long extraction of a traditional Vietnamese drip filter, leads to a very strong, bitter brew that stands up well to the cloying sweetness of the sweetened condensed milk. Chickory provides that same bitterness, which is why New Orleans coffee is often basically half coffee and half milk (and usually with some sugar too). If you try to make either New Orleans or Vietnamese coffee with an Arabica, even a strong, dark roast, it just doesn't have the bitterness you need to make it taste right.