YITH Waitlist helps you not miss sales of out-of-stock products by letting your users subscribe and get notified as soon as the product is back in stock.
When used with YITH Multi Vendor, also your vendors will be able to get email lists for their out-of-stock products and inform their potential customers when the item is back in stock.
To use this feature, make sure you install and activate both plugins. Then, select Manage waiting lists for their products in the option Regarding his/her orders, a vendor can, available in YITH > Multi Vendor > Vendors > Vendors permissions.
Now, whenever a product is out of stock, the user will be able to see a box on the product page to enter his/her email (or a checkbox if already logged in) to subscribe to the waitlist for that product.
Also, vendors will be able to see a Waitlist menu in their dashboard and check all products that have an active waitlist and the users in it, send emails, delete users and so on.
For more details about how to use YITH Waitlist, please, refer to the official plugin documentation.