Or press "B".

Almost all menu options in Vegas Pro have a shortcut, but the worst thing is having to remember them all - which often makes the menu options more accessible. For a list of all the shortcuts (dozens upon dozens of them), go to the Help menu and select 'Keyboard Shortcuts'. The menu items such as 'Audio Bus Tracks' have the shortcut listed against that menu item, but you're already there for the checkbox - I imagine that the shortcut is listed in the menu item for future use.

@Dexcon, I agree. But many people have pressed "B" by mistake and gotten it to show up. So knowing that is the keypress used can help prevent that in the future or realize it was an accidental keypress when it does appear.