Kindly someone let me know why one of my Vanda having this issue ? and what should i do ?

Thanks a lot

Ohh, that doesn't look good. Is the center growth still firm and green all the way down in the center?

Your Vanda is in deep trouble. If it were a mature plant you could cut out the brown leaves until you get to good tissue either above or below. But in seeing how close that the rot is to the root system, it has probably already progressed into the roots.
If you have enough growth above the rot you could remove all leaves until you get to completely healthy ones. Then cut back the core, stalk, to 100% clear tissue. Treat with cinnamon. If there is enough plant remaining, it could form roots in a couple of months and hopefully survive.

If you are intent, upon saving it, try using some long fiber moss & fill that basket, with it as much as possible, then keep misting several times a day, providing good air circulation & plenty of bright sunlight, but under shade of a tree.
If you can use diluted fertilizer regularly, with frequent heavy watering, you may just get roots to form above it & then once they've grown, it may "outgrow" the area that appears to have some sort of problem.
Then, once new roots have appeared to be growing well, above that area, cut with a clean & very sharp blade, carefully. just above the bad looking section.
Keep away from any other orchids & if it gets any worse & you have other orchids, you may not want to risk, causing any others to get the same problem.

If the last growth in the center is rotten, it might be beyond saving. That's why I was asking!
But, I have seen dying vandaceous Orchids growing a keiki at the bottom of the plant .and surviving that way.

The last thing that you want to do in my opinion with a Vanda, or any orchid for that matter, suffering from rot is to increase moisture surrounding the plant. Sphagnum moss will increase the moisture, misting it often will increase moisture and fertilizer will only increase the stress on the plant.
By getting rid of the rot, you increase survival of the plant. By keep it dry, you promote new root growth. You keep it shaded and out of direct sun in order to conserve what moisture it has left internally.
Good luck and let us hope it survives.

Thanks a lot for all the suggestion, I have cleaned the area bit, some roots are still growing. Do I need to isolate this from rest of the orchids ?

I would isolate it just to be safe.