Diploma

Yes, you can go for MBA as you have already spent lot of years in studying B.Tech. Doing MBA will enhance your management skills.

Major difference between MBA and PGDM is as follows: MBA is offered by 'Universities' or 'Colleges that are affiliated to Universities' or Central Body directly under the Indian Government while PGDM is offered by individual colleges and IIMs.

No, for post-graduation, graduation is mandatory.

Hello Saba,
Yes, you can. Eligibility for undergraduate courses in IGNOU is normally 12th. I am sure that you have passed 12th hence you should not have any trouble in getting admission. You can always check the eligiblity, if you still have doubts, in the downloadable brochure of IGNOU.

Yes, you can apply for B.Tech course in VIT.
VIT conducted their own entrance exam for B.Tech course.
Those candidates who have completed diploma course from a recognized university are eligible for B.Tech course in VIT.
Your age should be above 17 years.

Yes, after completing your engineering Diploma, you gets direct entry to 2nd year of engineering. Some colleges take admission through lateral entry exams like Joint Entrance Examination of Lateral Entry (JELET), Diploma to Degree Entrance Competitive exam, etc. So, you need to appear and crack these exams. Some colleges conducted their own entrance exam for getting entry. Admission procedure varies from college to college. So, you will have to contact colleges individually.