Another person had a system for learning the tones of Mandarin, Hmong, and Thai that involved practicing all of the possible two-tone combinations. He did this mainly to help in accurately perceiving and producing the tones; along with knowing each tone in isolation, you eventually have to know the transitions to and from every adjacent tone.
- if you have some data source (or access to a native speaker, or some
$$$), this is a low-hanging fruit for a #building-language-tools project ( #project-listen-to-viet now existing ) - see also: ~ minimal pairs utilize brain's capability of picking up complex pronunciation rules