In general, any cup (250 ml) of black coffee without sugar, milk, creamer, and other additives contains 2 calories.
Once you add a tablespoon of sugar to it, your coffee cup will have 18 calories. So, such ingredient as sugar adds 16 calories to it.
To make real Vietnamese coffee, you will have to use condensed milk and add it to your coffee. Depending on sweet you want coffee to be and how much condensed milk you add, content of calories will increase a lot. For instance, putting 3 tablespoons of condensed milk will add about 113 calories per serving.
All in all, a cup of Vietnamese coffee contains nearly 115 calories without sugar and nearly 130 calories with it.