Max chars is 30.

The width of 220 is about the width of the largest last post table cell.
The smallest one I currently have showing is about 150px.

I'm suggesting a larger width just to see if it will work. It looks to me like the table width is being affected by the side of the max characters. So, if you make the width way larger than that, you can see if the width is working or not.

I set it to 300 now. The tables do not expand. :(

In your header (thead) where it lists Info & Intro, Last Post, etc, you are setting the width for the column Info & Intro to 100% and that is interferring with your Last Post width. Get rid of the 100%:

I removed the width of 100% and the table cells expanded.
But now I have an unwanted effect in the whole table cell. It seems the attribute width="100%" in the table tag is not followed.

Okay, cool. I checked the markup on vbulletin.org.
With using relative widths in the thead cells, I was able to fix the problem.