Sidebar vertical line

There are 2 ways:

1. Adding border to post part or sidebar

2. using a background image (with a line that separate sidebar from post) for the content area

Second option is better in appearance, because the first option shows the line till sidebar ends (if you have added border to sidebar) or post ends(if you have added border to post area).

https://www.alistapart.com/articles/fauxcolumns/

Try this!