If you get the slightest feeling that your client's vision is nothing more than a "fog of war", than drop it.
Forget about it!
Politely step off, and put time shortage upfront as an excuse.
Eventually, what will happen is you will be spending 6h/night, 7 days/week, 3 different versions… all in vain.
You know better than to waste your ideas on confused people.
If the idea is brilliant, make your own product.
Projects are all about specs. They tell you what to do, you do it, they pay the labour. It's that simple.
Ok, so you still want to help people out and make the Internet a better place?
Then charge for consultancy.
You're more than a webdesigner!