Root is the root Viewport (one level above whatever you add to the scene). In your case "Node" is not the root of the tree, it is the root of your scene. So if you want to use that same code, you have to start with "Node", not "ViewportContainer". Another way is to use an absolute path:
get_node("/root/Node/ViewportContainer")
You can also do this, which makes things easier if you rearrange the tree:
get_tree().get_root().find_node("ViewportContainer", true, false)
I would still recommend the first code as it is shorter, but the second can help in cases where you want the tree to be flexible or you are moving things around.
