In X-Cart Marketplace, you can use the Messages add-on to enable direct communication between the store customers, the sellers (vendors), and the store owner/administrator.
The usage of the Messages add-on is described in the Messages add-on manual. Here we will go over the multi-vendor-related specifics.
First of all, as you probably remember, a multi-vendor store/marketplace at any given moment functions in one of the two supported multi-vendor modes: "Vendors as separate shops" or "Warehouse."
In the "Vendors as separate shops" mode, the user who handles orders and ships the purchased products to the buyer is the product owner (a vendor or the store administrator acting as a vendor). So it is only natural that customers' messages about products and orders in this mode should go directly to the product owner. In accordance with this approach, the Messages add-on organizes messaging in this multi-vendor mode in such a way that when a customer chooses to write a message to the store staff about an order, this message goes to the respective vendor. The store owner or administrator can supervise this communication as they can view all messages between customers and vendors. Still, they only interfere in the communication process to handle disputes or to help resolve problems that a vendor cannot resolve by themselves.
In the "Warehouse" mode, the products all come from different owners (vendors). However, a customer buying products from different vendors still gets a single order, and all the products are shipped to them from a single warehouse location. When running a multi-vendor store in this mode, different businesses will have their preferences as to who should handle communication with the customer. The Messages add-on can adapt to different business needs as it enables the store administrator to specify who should run the communication with the customers.
On the add-on settings page, the administrator can enable the "Allow direct communication between vendor and buyer in the "Warehouse" multi-vendor mode" toggle:
If the administrator is the user who will handle all the communication with the customers, they should keep the above said setting disabled. In this case, all customer order-related messages will go directly to the administrator; the vendors will not have access to them. If, however, the administrator wishes to enable the vendors to handle the communication with customers by themselves, they need to enable this setting. In this case, order messages from customers will be addressed to vendors. If an order contains products owned by different vendors, the customer will have to choose which vendor they wish to contact.
In addition to order messages, X-Cart multi-vendor stores in the "Vendors as separate shops" mode or in the "Warehouse" mode with the "Allow direct communication between vendor and buyer in the "Warehouse" multi-vendor mode" toggle enabled, support messages not related to orders. Any customer can contact a vendor in such stores using the Contact now link on the vendor page on the storefront (no purchase is required).
Unlike communication threads about specific orders, threads non-related to orders are marked as "Conversation":
The store administrator can view and manage the communication treads of the "conversation" type in a way similar to order related threads: