only one vendor's items in the cart at once- one vendor checkout. (haven't tested yet 8/22/2016 )

gistfile1.txt

    //Order only from 1 shop 
//https://www.wcvendors.com/help/topic/restrict-clientbuyer-to-order-from-one-vendor-at-a-time/
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[] = $values['data']->post;
    }
    if(isset($_product[0]->ID)){ //getting first item from cart
    $product_in_cart_vendor_id = get_post_field( 'post_author', $_product[0]->ID);
    $prodId = (int) apply_filters( 'woocommerce_add_to_cart_product_id', $_GET['add-to-cart'] );
    $product_added_vendor_id = get_post_field( 'post_author', $prodId );

    if( $product_in_cart_vendor_id !== $product_added_vendor_id ){$woocommerce->cart->empty_cart();wc_add_notice(  __("You can only order items from 1 shop !", "wcvendors"));}
    return $cart_item_data; } 
}