Preparing for the GRE Mathematics Subject Exam?
I am a student at a "Top 10" applied math program in the US. For us it was not the case that we needed top scores of the math gre for admission/fellowships (thought this was not the case for my pure math cohorts). So maybe this will help with the stress of the exam.

To answer, since you say that you are the weakest on vector calculus, I would spend the most time reviewing this materiel. Calculus makes up a majority of the questions and from my experience these questions were either you knew the "trick" or you were going to lose a lot of time doing it from the fundamentals.

Since you say that you are going to take it twice, here is my biggest advice. Try your best to remember the questions from the first exam. Especially if you don't know how to answer them. I also took the exam twice and ended up getting a couple of questions that were identical. So when you are done I would try to write down as many questions as you can remember to learn how to do for the next one.