how much marks one need in viteee to get a rank under 1000 and under 100 respectively, give a rough estimate. thank u

This kind of predictions can't be made as everything changes from time to time. So you should try the best you can do rather than to follow any data based predictions.  Still for your preference you can see the data provided below:

Marks                                   Rank
115-119                              250-500
90-114                                500-2501
80-90                                  2501-5000
70-79                                  5000-6500
60-69                                  6501-8500

I hope my answer helps you. All the very best for your future endeavors!