If you have added a vendor via the Contacts > Vendors page, you can add a property for sale to their record.
When looking at the vendor record, click the “Add a property” button in the Properties for sale widget. Fill in the form displayed as far as you can and click the Proceed button. This will add the basic property details to the widget.
You can then click the pencil icon next to the property to enter the property edit screen where you can add images, pricing details, description, etc.