Well a website can be "simpler" or "complex" but what i tried said was when you got some buttons and a main "container" that will have the contents for the buttons in this case i use a viewstack.
About the second question, imagine that:
A Module that have the recently news and the most view news, in this case i will use a view stack but you can load it to your application and put it on a side bar or other place you don't have to put this module in a view stack, my english is not good and sometimes is hard to me to explain, do you understood what i am trying say?
Off course you can mix, you should think in your projects on a simple way, try to make separated modules that can be reuse in other projects and can be totally independent of other Modules.
Sometimes you will have one, two,three viewstacks inside a module others maybe you will not use it, like i said it always depends on project
Sorry my english