Find this line:
$InsertQuery->saveInSession("");
$InsertQuery->execute();
$InsertGoTo = "list-vendors.php";
and update it to:
$InsertQuery->saveInSession("");
$InsertQuery->execute();
var_dump($InsertQuery);
die();
$InsertGoTo = "list-vendors.php";
That will print information about the insert onto the page and might give you information on how or why it fails.