When Is It the Right Time to Focus on Vocabulary?

I am doing an action research project with my students the last two weeks of school. What I want to learn is when is the best time for students to spend time with vocabulary from a textbook.

I would think that learning the vocabulary words first would be more helpful when it comes to understanding the text. I also think their is a possibility that going over the words at the end may make the students' short term memory work better for the test. I also want you to note that this assessment, like most textbook assessments, is heavily weighted with vocabulary questions.

Here is my procedure: I have three different fifth grade science classes so I split them into three groups. The first group is a control group where I will teach two sections in our science book without a special focus on vocabulary, just the normal worksheet provided in the student materials. The second group will begin the short unit doing two of the vocabulary skills outlined in Allowing Students to Differentiate with Vocabulary Strategies. The third group will end the unit with doing two of those vocabulary skills.

Ultimately, this small sample won't completely answer the question, but it may give me some insight into this. Would you teach a unit's vocabulary at the end if research shows the students would score better on the test that way? Or would you teach the vocabulary at the beginning?