Using the ReCaptcha for vBullentin is quite vulnerable to your forum as spammer has figured a way for the robot to bypass your SPAM detection.
A newer way of preventing spam-bots from registering on your forum is to use KeyCaptcha. Instead of the user having to guess the distorted words in reCaptcha and type in the text, with KeyCaptcha the user are required to complete a simple task by completing a simple puzzle image. They will need to use a mouse and drag the missing pieces from the stack to complete a full image. If you are willing to pay for the premium service you could remove the Free Version background image and additionally you can customize your own puzzle images.
1. First you need to sign up with KeyCaptcha and download the require file for vBulletin. As of this writing the current version is keycaptcha-for-vbulletin-37-4x.zip.
2. Once you have completed the registration, you should be presented with a Wizard to allow you to download the right file.
Fill in all the necessaries information in Step 1, 2, 3 and 4.
3. On the last step you should be presented with a detail instruction how to install this product into your vBulletin forum.
4. Once you have downloaded the zip file, you need to unzip it, inside it there should be two files. One is class_humanverify_keycaptcha.php and the other is keycapcha-for-vbulletin.xml.
5. Upload the file class_humanverify_keycaptcha.php to the includes folder in your vBulletin directory.
6. Next, login the vBulletin Admin Control Panel and Expand Plugins & Products on the left hand side navigation. Then click on Manage Products.
7. Click on the [Add/Import Product] link at the bottom.
8. Then under Import Product section, click on the Browse button and select the file keycaptcha-for-vbulletin.xml.
9. Now click on Import button.
10. Now go to Settings > Options > In the list select KeyCAPTCHA Settings.
11. Now finally put in the Private key that you were given at the registration process on the KeyCAPTCHA website.
12. Click on Save button.
13. Next go to Settings > Human Verification Manager.
14. Now choose KeyCAPTCHA from the list of radio boxes, and click on Save.
15. Finally you can go and test out the new registration yourself.