If there are no vacant seats in that particular year then what happens

Hey there!

Since you have got a preparatory rank in JEE Advanced, you are eligible for taking up the preparatory course offered by various IITs, which covers the subjects, math, physics, chemistry and English. It is for one year after which you are declared as either "PASS" or "FAIL". If you successfully pass the course, you are eligible to secure a seat in the respective IIT, based on the vacancy in that particular year. If you are willing to go spend an extra year to get a seat in IIT (considering you pass), you should go ahead and join the preparatory course, or else, you can go for the NITs or other GFTIs which you might be getting with your JEE Mains ranks or any other college based on the rank secured by you in their respective entrance examinations. 

Hope this helps. All the best!