The onus for protecting your server is on you. If your machine is compromised at the system level, there is nothing you can do to vBulletin to stop this.
What "attacks" are you considering?
There're two levels of protection. On the server, it's our hosting service provider who take care of this. But if we use vBulletin, it's on this product level that the security settings must be made. I've some friends using vBulletin and their site were attacked by some hackers, using some shells installed via files/images upload way. With these shells the hackers can take admins' passwords, or replace the index files, etc. Does it mean some product apertures? So, I'd like to know if, when I buy your license, you've got methods for protection from this way of attack, or are there no such apertures in your official products...