I am not a template (HTML) developer at all, so i might be wrong, but it seems you set a fixed width in your template.
In the template you have posted in post #3, you have the following line:
HTML Code:
<td class="alt2" width="175" style="border: $stylevar[cellspacing]px solid $stylevar[tborder_bgcolor]; border-top: 0px; border-bottom: 0px">
I think the width="175" is what gives you this problem.
PS Your template does not look like a default template at all. Which vBulletin version are you using?