I was going to write how the pitch changes in 6 Vietnamese tones, but a quick search on Google reveals this scientific research on the same topic. It’s fascinatingly correct! Go read it if you are interested in the topic–which I assume you are since you are here.
We see that it’s not just pitch differences between the tones, there are pitch difference in the tones themselves. Imagine how hard it is to write lyrical music in Vietnamese, let alone improvisational singing.