If you want your students to learn vocabulary and actually remember it, prepare a vocab jar. After each and every class, put new words in a form of flashcards into the jar. At the beginning of the next class, ask a student to choose two flashcards from the jar, to check if he/she learnt the new words. You may have a grading system like this: for two correct words a student gets a plus (+), for two incorrect a minus (-), and for one correct and one incorrect a student gets nothing. Students collect pluses and minuses and they exchange them for grades (e.g. 5 pluses = A; 2 minuses = E).
My teacher used to have this system and it worked perfectly.
Remember: You add vocabulary to the jar but you do not change it. This way, students always have to know and revise all the words.