Firstly, congratulations on securing Mechanical at VIT. Almost exactly four years ago, I was standing right in your shoes and I probably knew way less than you know right now. Unfortunately, I am not sure that whether you are about to join VIT Vellore or Chennai, so I am going to give you an answer pretty much applicable to both.
In both colleges you will find dozens of clubs devoted to various fields associated with mechanical engineering. These clubs could be related to anything from Automobile Engineering to Aerospace Engineering. Being such a versatile branch, Mechanical Engineering finds application in many fields and hence the numerous clubs you can join.
You will be eligible to join any club in both campuses considering you meet any sort of criteria that the club might or might not chose to put forward. The difficult part is to choose which ones to join. The initial feeling would be to join as many as possible to get a taste of them all. This is exactly what I did as well. I soon found out that it was impossible to give my best to more than 2 or so clubs at the same time.