This page has several references to a paper I have given on the importance of an early emphasis on vocabulary.
The main paper is here
The table of what learners can do with X words is here
The table of a syllabus one might follow is here.
The most important 2000 words list is here
The receptive vocabulary test is here. The productive vocabulary test is here.
There is no answer sheet for these tests!!! I'm sure you don't need it. (Your students may find it!!)