I believe that your Dealership is doing exactly like
@Jstump99 is telling you, they are stringing you along. The reason why they want you to get a VIN because at that point if you were to cancel your order the Dealership still gets that vehicle, however if you canceled your order with only having an Order in with that Dealership, they will have to cancel and they get nothing....
You need to go to that Dealership and ask (maybe even demand) that you want a Signed Purchase Agreement or you will go somewhere else once you cancel your order. Then see what they have to say, if they give you all kinds of excuses, tell them your a Member on Bronco Nation and they told me exactly what to do, say to them "your not going to like it when they're done with you" that will get them thinking.
I know for a fact that you can get a Signed Purchase Agreement when you are done ordering your Bronco.
Good Luck to you!