Since V3.9.7 of the Fancy Product Designer WordPress/WooCommerce plugin, you can use Dokan to let your vendors sell customizable products created with Fancy Product Designer.
You only need to uncheck the option "Admin area access" in the Dokan General settings. This way the vendors in your site can access the WordPress backend.
After that they have access to Products and Product Builder page of Fancy Product Designer.
Under the WooCommerce tab of Fancy Product Designer settings, you'll also find a new section Dokan. Here you can also toggle the Fancy Product Designer menu in the backend for vendors and select the FPD products from a user that you will look to offer your vendors to choose from. That way you are able to offer a set of products your vendors can only select from instead of creating own FPD products in your system.
Whenever a vendor receives an order, he is able to view it by clicking the "View Customized Product" button in his Orders section. Then he'll be redirected into the WooCommerce Order Details of the backend.