I see some that do, but I never really join many vbulletin forums at all because of that reason. It's only because of not much security for thier register, and stuff like that. If they had more security it would be alot less. Like recaptcha, anti-spam questions, and so on.
The reason for it is because they have a very easy default Captcha system by default and the spam bots cracked it years ago
I agree spammers try to target vb forums more because they know they can most likely get away with it. Vb forums have really outdated spam tools and even the plugins are useless these days.
Not every Vb forum I join/Frequent is covered in spam but yeah I do see a vast majority being targeted by waves of spammers, Vb lacks a lot when it comes to spam management.
I used to have a vbulletin I would have 300 people on my forum. Then when I get home from school I have about 450 spam bots so I would take all my time to do it. Then XF came and I switched.
I use vBulletin and I rarely get any spambots at all. I get around one every other day or so. But my security is high. Captcha until you reach 5 post count. Captcha and Security question when registering.