I am having the same problem, I cannot choose vendors. This might be because of the two errors showing up in the console.
1. product.js?ver=2.3.9:7 Uncaught TypeError: $(…).ajaxChosen is not a function
2. wp-content/plugins/dc-woocommerce-multi-vendor/assets/admin/js/admin-product-auto-search.min.js?ver=2.3.9 404 (Not Found) 404 (Not Found),
The min version of the mentioned js is not in the plugin directory
