On the placement test, is the emphasis on kanji or is it more grammar and vocabulary?

All three skills are important.
The placement test doesn’t include an independent kanji test, but since the test questions (and your answers) are in kanji, you will need to know some kanji.
Grammar and vocabulary of course tested. For you to understand each question, you need to have both grammar and vocabulary. If you have enough kanji and vocabulary skills to read the questions, you will have the grammatical skills to answer questions properly. You will also have listening tests and an interview with a teacher.
And, after all, we’re trying to determine what you understand. If you don’t respond perfectly to the entire test, we probably won’t place you in the most advanced level.

To preview sample compositions composed by students towards the end of each level, please download Language Course Equivalencies.
For best results, study all three—kanji, grammar, and vocabulary. Brush up on both written and oral skills.