How to buy products from a single vendor. No orders with different vendor products

You can allow to buy products from a single vendor. No orders with different vendor products with this custom code in functions.php of your theme:

if( function_exists( 'YITH_Vendors' ) ){
    add_filter( 'woocommerce_add_to_cart_validation', 'yith_wcmv_multivendor_cart_validate', 30, 2 );

    if( ! function_exists( 'yith_wcmv_multivendor_cart_validate' ) ){
        function yith_wcmv_multivendor_cart_validate($valid, $product_id ){
            $current_vendor =  yith_get_vendor( $product_id, 'product' );
            if ( ! $current_vendor->is_valid() ) {
                return $valid;
            }

            $contents = WC()->cart->cart_contents;
            if( !empty( $contents) ) {
                foreach ( $contents as $item_key => $cart_item ) {
                    $vendor = yith_get_vendor( $cart_item['product_id'], 'product' );
                    if ( $vendor->is_valid() && $vendor->id != $current_vendor->id ) {
                        $message = 'You cannot purchase product from different vendors at the same time.';
                        wc_add_notice( $message, 'notice' );
                        return false;
                    }
                }
            }

            return $valid;
        }
    }
}