Nankai's vocabulary is CET-6, Peking University's vocabulary is around 8000, and Xiamen University's vocabulary is relatively simple.
Testing vocabulary mainly depends on the depth of vocabulary, generally only 5500 words. I suggest you use Band 6 to review your vocabulary.
Generally speaking, it is not difficult to take an English test because you have to take care of some doctors on the job. 、
You'd better review according to the school you want to apply for and buy the real questions of the school over the years.