I tested this on another v-Bulletin forum and I doubt this version is any different. PMs from people on your ignore list will not show up in your message box. You won’t get a notification and you won’t know they sent you a message at all. The person who sent the PM won’t realize that you didn’t receive the PM, from their end it will appear that the PM was sent, but the PM just floats off into the internet ether.
If you have your PMs disabled instead, no user will be able to send you a PM, the Send a PM option won’t show up in the drop down menu for your name (in a post) or at the top of your profile page.