If the scope is not clear enough for putting together a statement of work of acceptance criteria as described in other comments, one thing that you could do is to start with a time-boxed contract, ranging from a couple weeks to a couple months, depending on the complexity of the project, to do requirements analysis. The deliverable of this contract would be a list of functional and non-functional requirements, at a reasonable level of detail.
The issue is that, for bigger projects, you start running into the problems related to a "waterfall" approach. As an alternative, you could look into how "agile" organizations approach their contracts -- basically by selling the ability to work on "stories" for as long as the customer wants them to -- and the deliverables for each story are very fine-grained.
Either way, without active involvement of the customer to either elicit requirements up front or validate your stories as you go, you'll be in for a rough time. It's probably better to have an honest discussion with them about this, because it's in their best interest after all.