From the buyer's end, what's notable in both cases described here is that the customer asked the vendor to sign the NDA after having already divulged the projec

From the buyer's end, what's notable in both cases described here is that the customer asked the vendor to sign the NDA after having already divulged the project scope and details. You're doing it wrong! The whole purpose of an NDA in these cases is to protect the proprietary information that is handed over by the buyer in the normal course of scoping and proposing a project. Karma and politeness aside, when you are out in the marketplace, you need to protect yourself.