"I would like a cake. How much is that?"
If you walked into a bakery and asked that question, you can bet your sweet tooth that the person behind the counter wouldn't respond with a price. He'd probably ask several questions: what size? how many layers? what kind of frosting? what kind of cake? do you need it decorated with someone's name? do you want it now or later?
And if you said, "I dunno. Just a cake. You know, tasty? for eating?" and the bakery gave you an estimated price, they'd be guessing. If you then walked into another bakery and asked the same question, with the same (lack of) clarifications, and that second bakery gave you an estimated price, they'd be guessing too. If those prices were different, you wouldn't have any real way of knowing whether the cheaper price was for a smaller cake, a simpler cake, or the same cake at a better price. No one had any idea what you wanted...
...so you probably won't get what you wanted.
This is not different with a web site project. Coming up with a simple list of requirements for the project you want completed makes good sense, not just once you've chosen your vendor, but before you choose your vendor. There are several advantages to providing vendors with a list -- just a page, nothing fancy -- of what you want your site to look like, contain, and do, and any special concerns you have before you start.