Explore the Requirements Space
Vague requirements are a waste of time and money. The core team needs to help the organization define the overall project scope and explore the requirements space before attempting to create the overall project plan. Good communications skills are needed at this point to ensure that all of the various stakeholders are involved. Use prototyping, storyboards, use cases, user stories, feature lists, or whatever to get the organization involved in figuring out what's needed and defining a clear vision for the application.
My personal preference is toward using use cases for this activity because they can be written at increasing levels of precision as more effort is expended at understanding the requirements. Indeed, I usually suggest that the entire development team should know how to write use cases, but the key thing is to have an active, ongoing conversation between the developers and the stakeholders. Nothing is more demoralizing for developers than to realize that the application they've slaved over is technically correct but irrelevant because it solves the wrong problem.