With rank 20700 in vitee, which categories I should apply for CSE or specialisation in CSE.

As per the previous year's cut-off rankings, you have a good chance of getting CSE (probably in category 2) or CSE with a specialization or IT in VIT Vellore. For CSE it is best to have a rank below 5000. After 5000, it all depends on the seats available and how high is your rank. The farther your r