Hola Villa
Debes utilizar una funcion top, max, min U_vinculacion en la subconsulta o modificar el query de la siguiente manera.
SELECT T0.DocDate, U_idcliente, T0.CardName 'Nombre Empleado', T1.Quantity,
T9.U_vinculacion 'Vinculacion', T0.U_secretaria 'Secretaria',T0.U_telcliente,
CASE T1.ItemCode WHEN '13' THEN 'Explora' ELSE 'Planetario' END 'Boletas'
FROM ODLN T0
INNER JOIN DLN1 T1 ON T0.DocEntry=T1.DocEntry
LEFT JOIN
(SELECT U_cedula, U_vinculacion FROM [@PET_EMPLE_MUNICIPIO] ) T9
ON T9.U_cedula = U_idcliente
WHERE T1.ItemCode IN ('13','32') AND T1.U_LISTA_PRECIOS IN ('22')
AND T0.DocDate BETWEEN '01/06/2018' AND '30/06/2018'
Saludos,
Andres Ramirez Jaramillo