[Lautaro]

1- Go to www.recaptcha.net create an account and add your forum link.

2- copy the private and public key given after adding your forum link.

3- Go to your forum's AdminCP > vBulletin Options > Human Verification Manager :

choose "reCAPTCHA? Verification" and click "SAVE" then you will see 2 input spaces, one for the public key and another for the private key, add the keys you copied over there and click "SAVE"

4- logout and try to register, check if everything works fine.

you'll never have a bot registered in your forum again.

I hope it helps.