Combine egg yolk with condensed milk and vanilla extract in a small glass jar and whip with an electric frother until light and frothy. This may take a few minutes.
To test whether the egg yolk is light enough, take a small spoonful and drop it into a glass of water. If the egg mixture floats, it's good to go. If it sinks, whip the egg yolk for a little while longer.
Add the hot brewed Vietnamese coffee to a heat-safe mug or glass and gently pour the egg cream mixture on top.