Google yielded two answers. But Logan R. Kearsley, MA in Linguistics from BYU is too recondite. Can you please explain at the level of a 16 y.o.? What does “analytical level of a language that deals with vocabulary” mean?
A lexicon is a list of the root words (lexemes) used in a language (“run” but not “running” or “ran”).
If you analyze a complete language and list every word ever used - you have created a “lexis” of that language. A lexis is the list of ALL words in a language and how they have been used (“run”, “running”, and “ran”)
A vocabulary is a limited word set within the lexis applicable to a particular context or use.