I received a good bit of advice from our VP in IT, which was to only contract for the Discovery & Define phase and then decide if that vendor was right for the implementation.
For me, Discovery & Define was critical to align the businesses (we have five sales groups) and gain consensus on key elements such as goals and objectives.
We conducted a day-long exercise with marketing executives including some break-out sessions by business. Getting the information by business and then pulling it together revealed that the business challenges were not unique, but shared. It also demonstrated that the goals and audience target were similar.
The break-out results removed any feeling that decisions were made to benefit one business at the expense of the other.
Throughout the session we emphasized that the decisions and guidance given would be used as we defined the navigation and wireframes. We created a discovery document that outlined the results of the session, circulated it for review, feedback, and approval, and then used it throughout the define period.
Now, if someone asks why did you do this? I can pull out the book and say because the businesses said…
The other benefit of this approach was that we limited the time required of marketing to the one day session, plus review of the doc, and maybe a few one-off questions as needed.
The document, site navigation, and wireframes were then provided to each vendor being considered for implementation. We did offer it to the company that conducted the Discovery & Define, but another vendor ultimately got the contract for the implementation.