functions.php

// Woocommerce 
// Restrict customer to buy in only one shop
add_filter( 'woocommerce_add_cart_item_data', 'woo_custom_add_to_cart_with_variations' );
function woo_custom_add_to_cart_with_variations( $cart_item_data ) {
	//Compliant with product-variations
	//Tested with woocoommerce 3.1 | WC Vendors PRO 1.4.3
	global $woocommerce;
	$items = $woocommerce->cart->get_cart(); //getting cart items
	$_product = array();
	foreach($items as $item => $values) {
			$product_id_in_cart = $values['data']->id;
			$_product[] = $values['data'];
			break; //after 1st product we can stop the loop
	}

	if(isset($product_id_in_cart)){

		//GET VENDOR ID OF FIRST PRODUCT IN CART
		$product_in_cart_vendor_id = get_post_field( 'post_author', $product_id_in_cart); //get vendor id from cart.

		//GET LAST PRODUCT ADDED TO CART 
		$product_id_added = (int) apply_filters( 'woocommerce_add_to_cart_product_id', absint( $_REQUEST['add-to-cart']));

		//GET VENDOR ID OF LAST PRODUCT ADDED IN CART
		$product_added_vendor_id = get_post_field( 'post_author', $product_id_added );

		//CHECK IF BOTH VENDOR  IDs ARE EQUAL
		//TODO : ask user if he wants to empty his cart.
		if( $product_in_cart_vendor_id !== $product_added_vendor_id ){$woocommerce->cart->empty_cart();wc_add_notice(  __('You can order only from 1 restaurant!', 'swiss_takeaway'),'error');}
		return $cart_item_data; }
}

Dear Marc, The codeing is very useful. but the error message is not displaying for me. Can you help me. I using Wc vendors version Version 2.1.0 and woocommerce Version 3.4.4