MattsStuff

add_filter( 'woocommerce_add_cart_item_data', 'woo_custom_add_to_cart' );

function woo_custom_add_to_cart($cart_item_data) {
	global $woocommerce;
	$items = $woocommerce->cart->get_cart(); //getting cart items
	$_product = array();
	
	foreach($items as $item => $values) {
		$_product[] = $values['data']->post;
		var_dump($values);
	}

	if(isset($_product[0]->ID)) { // Getting first item from cart
		$prodId = (int)$_POST['add-to-cart'];

		$product_in_cart_vendor_id = get_post_field( 'post_author', $_product[0]->ID);
		$product_added_vendor_id = get_post_field( 'post_author', $prodId );

		// Debug check if numbers are actually appearing
		echo "product_in_cart_vendor_id" . $product_in_cart_vendor_id;
		echo "product_added_vendor_id" . $product_added_vendor_id;

		if($product_in_cart_vendor_id === $product_added_vendor_id) {
			return $cart_item_data;
		} else {
			wc_add_notice( 'Whoa hold up. You could only add items from one vendor at a time in your cart', 'error' );
			$woocommerce->cart->empty_cart();
		} 
	}
}