1) If you don't want to display Visual Composer elements in your sidebar go to Appearance > Sidebars Editor select the desired sidebars and click the green "Override widgets" button so it turns grey. This is essential if you're just adding normal widgets and can't see them in your sidebar/footer areas.

2) If you want to display only Visual Composer elements in your sidebar go to Appearance > Sidebars Editor select the desired sidebars and click the green "Override widgets" button so it turns green if it's not already enabled.

3) If you want to display both Visual Composer elements and ordinary widgets in your sidebar go to Appearance > Sidebars Editor select the desired sidebars and click the green "Override widgets" button so it turns green if it's not already enabled. Set the Override behavior to "Place before existing widgets".