AdminCP -> User Profile Fields -> Add New User Profile Field

Profile Field Type: Single-Line Text Field

This is an example, so feel free to change any of the following values when creating your User Profile:

Title: Can you spell?
Description: Enter the first letter of the word “Soccer”
Default Value: Z
(Anything but the valid answer will suffice)
Field Required: Yes, at registration and profile updating
Field Editable by User: Only at registration
Private Field: Yes
Field Searchable on Members List: No
Show on Members List: No
Regular Expression: ^[sS]$
(This expression only allows an ’s’ or ‘S’ as valid answers, adjust accordingly to your question)

Hopefully by taking the above steps the amount of automated registrations and spam on your vBulletin forum should decrease.

Quick Update: Since implementing this on my Forum, I have yet to see a single automated registration bot make it through the registration process.