Sorry for the late response. As I mentioned earlier, our sister company GroupDocs provides such solutions. Please have a look at http://groupdocs.com/apps/assembly#
You can contact GroupDocs support at http://groupdocs.com/ for further information. I had a discussion with GroupDocs representative and he told me that they also have a plugin for Magento. So, you can create and send filled order forms to vendors, vendors can fill one or more fields (ship date etc.) and updated data can be sent to Magento.
The only requirement is, you will have to prepare the document for Assembly (i.e. purchase order in your case) in groupdocs.com. Once the form is prepared, you can add embed the form to Magento using their plugin.
Please contact GroupDocs and discuss your requirement in detail with them.
Support Developer, Saaspose Sialkot Team