Ian,
This post is especially true to me as I make it through a digital project of my own. I think that flushing out the user experience is key to providing that understanding to a third party vendor. And I don’t think its going overboard to go over objectives for each page of a website or each module of an information system during initial conversations with a third party vendor. The key is know what you want, and then making sure that the vendor is on the same page.
-chris
