Suggestion: Motivate learners by showing them their vocabulary strength percentile rank over time among all users learning the same language.
A score could be computed in a way that rewards more words learned. The percentile rank would then be determined using that score.
A completed skill tree with full vocabulary strength indicators should give you the highest percentile rank regardless of the number of credits earned.
Learning vocabulary is the hardest problem when studying a language. I have gone through all the lessons on German. I understand the grammar pretty well, but every time I try to read some "real" German I am immediately stuck on some new word which sends me to a dictionary. However, there are a few very good dictionaries online. Nevertheless, it slows me down. Getting to be a fluent reader is going to take a long time.