I just posted a table in a post in my 4.1.11 test site and it shows the borders just fine. It's a default style and I have added no additional CSS to show it.
Create a new style with no parent:
- Styles & Templates > Style Manager > Add New Style
- Parent Style: No Parent Style
- Title: Default vBulletin
- Allow User Selection: Yes
- Save
Then browse the site using that totally default vbulletin style - do you still have the same problem with the table?