Oh, Thanks for pointing out that thing...
I do have a captcha enabled on the registration form but I just checked that the Captcha is NOT appearing in the form.
I checked VBulletin Options > Human Verification > It is showing everything as ON
Verify at registration: YES
I don't know what happened to the captcha....
Please help me.
Edit: OK, I figured it out myself. I had enabled the Question and Answer Captcha but there were no questions...
I have set it now at "Image Verification"... Let see if it solves the problem or not.