UCEED is the entrance exam if you wish to pursue design. Check details here https://engineering.careers360.com/articles/uceed. The next step would be CEED which is the PG entrance exam (https://engineering.careers360.com/articles/ceed). However, you can write CEED even if you are an engineer. Either way is fine to pursue specialization in product design.

in terms of job and money IT is better, but it depends on your interest and satisfaction

NTA Scores are given in percentile.
Percentile is calculated by the ratio of the number of values to the total number of values . You cant say/depict your actual score/rank from your percentile neither can you calculate it by only marks. To calculate the actual score/rank/percentile you would need to

Mechanical Engineering comes in group B.

Tuition fees for group B is INR 1,55,000

To check complete fee structure you can refer below link

VIT Fee Structure

Good luck!!

According, to 2017 data you should score more then 200 marks in JEE Main to get Mechanical or EEE branch in top NITs under general category other state quota.

JEE Advanced maximum are not defined :

Maximum Marks vary every year like below:

2017- 366

2016- 372

2015 - 504

If you think of getting a job in IOCL then you must have to clear GATE exam in that respective year.
However, you have to clear IOCL GATE Cut off marks for GD/GT/Interview for mechanical engineering is GEN(80Marks) OBC(75Marks) SC(65Marks) ST(60Marks) with respect to last year data.

Here is the

After diploma in mechanical engineering, you can opt for Btech in mechanical engineering, directly second year through lateral entry exam (LEET).

After earning graduation degree in mechanical you will eligible to apply for various private sector such as TATA Group, Kirloskar, Siemens, Geometric Ltd, Thermax etc. and government sector companies

All GATE papers consist of compulsory sections of Maths and General Aptitude along with English. It is advisable to prepare for these sections don't neglect these sections along with main syllabus. These are easier sections to score good marks in your exam. 

In order to start your preparation you should

All GATE papers consist of compulsory sections of Maths and General Aptitude along with English. It is advisable to prepare for these sections don't neglect these sections along with main syllabus. These are easier sections to score good marks in your exam. 

In order to start your preparation you should

If you score 240 marks in JEE main your approx. rank will be around 3000.

According, to 2017 data if your rank is around 3000 you can easily get top NITs in Mechanical branch.

You can also check the cut off rank from the given below link

https://engineering.careers360.com/articles/nits-cutoff-for-btech-mechanical-engineering

Good Luck!!