What to do about a bad, or nebulous brief
The very first part of any design project to identify the objectives and scope of the job in hand. If you don't have a concise brief from the client, write it yourself. If the objective is not clear, it is impossible to satisfy it. It may be that there is more than one objective, in which case you have to prioritise the objectives in a sensible way. It may be that you need to explore a number of confused or related objectives before deciding on which is the right approach.
Get the client to read your proposal, expand on it if necessary and agree to it. You might want to mention fees at this point too.