The simplest way would be to use CSS positioning. Do it in Dreamweaver or something first so you get the CSS just right, then copy it into your VB style.
Depending where you want the sidebar, you will need 2 DIV boxes. One for the sidebar, and another around the forum. Place the DIV code in VB header, and VB Footer. Place the CSS in the last box of your style "main vars" page. If you want it all centred you might need a 3rd DIV around both of the above; use float for the sidebar in this instance.
Again, depends how you want it laid out, but the basic answer to your question, I would say CSS positioning is the way to go. Although someone might have other suggestions.....