No. It is never enough to crack VIT by solving its sample papers. But if u work hard,U can crack VIT by just studying hard for a month.The examination questions are almost conceptual and memory based-questions that you can gather from your academic textbooks. 

.  Chemistry part will have many memory-based and direct questions. [NCERTs are enough for Chemistry].
.  This is an online exam without negative marking, be skipping the questions which you don't know and answer the well known questions and nearly-guessed ones.
. I suggest you to focus much on your favourite subject and be conceptually and numerically strong on that as much as possible for you. But don't neglect the other subjects.
.        The most IMPORTANT thing is that you should practise all the PREVIOUS PAPERS and take regular online mock tests.
.   Attend the exam without any fear, then you will definitely do great although you have much lesser preparation.
.     I hope the above things may definitely help in your preparation for VITEEE.

.     ALL THE BEST

NO, Completing the past year's papers and mock paper is definitely a plus point for any examination.This is a way to ensure and judge oneself as to how much you are prepared.Would like to add that it is definitely not going to decide your admissions because that is totally a different thing.

The questions that appear on VITEEE may be different from the ones you have solved previously, but they employ similar concepts to the ones you solved earlier. It is very important for an applicant to work on the concepts. Stick to the basics.

All the very best and good luck.