Enable image verification image in the registration form:
AdminCP --> vBulletin Options --> User Registration Options
Scroll down to "Image Verification", Ensure you can see the image saying "vBulletin" and if it is there click "Yes" and save it.
Possibly do an IP search on the account as it must have originated from a computer and if its a Unique IP ban it. Then delete the account so that it will be forced to register again and it will stumble across the image verification process and wont be able to register.