Nah, if they are determined, it won't, unless they want to flame and enter an invalid email, in which case you don't need them. When thinking about this question, consider "I want all links to open in a new window so they won't leave my site. . . " it's the same line of thinking.
The below are set up using the numeric "trivia" question I posed above, change to suit.
1. Log in to your vBulletin admin.
2. Open User Profile Fields.
3. Open link to Add New Profile Field.
4. For profile field type, select single line.
5. Set up the question.
For title, something like "Human Verification Question" should do the trick, in description, something like "To prevent automated registrations . . . "
Max length of user input: 1 or 2, depending on question.
Field length 3.
Required No, but display at registration.
Field editable by user: only at registration.
Private Field Yes
Field Searchable on Members List, Show on Members List both no.
Regular expression is the key. So if you ask "what is the product of three times two?" you would enter 6. Or being that it's a regexp, you could enter ^six¦SIX$ if you want to confuse everyone. :-)
Leave "show on page" at "Edit Profile" because by making it private it won't be seen.
Save it, test it, it works.
Original documentation from vBulletin [vbulletin.com]
More anti-spam measures [vbulletin.com]