Add a line from the parent tab to the last child while vertical tabs is enabled, why? Well I personally believe in clarity, and visually knowing if a tab is a child is rather easy, BUT, why not go the extra mile and make it super easy? See what I am envisioning is a grey line, that goes up and down, like it?
If not opening and closing Brackets work for me too "{" for the top level parent and "}" to close off the last child. But I think a line just looks better though...