Captch and reCaptcha are constantly being bypassed by bots these days so maybe install this
https://vborg.vbsupport.ru/showthread.php?t=254671 if thats the way you want to go, i have to tell you though that i've had the Q&A human verification thats built in to vbulletin in use since starting my forums a good few years ago, i have 10 questions that i change every two years and in all that time i've only had to ban 125 spammers all of which have been organic and not automated bots