SELECT MINDATA.VRDOCID, MINDATA.DATAINIZIO, MAXDATA.DATAULTIMA FROM
(SELECT MIN(VRDATA) AS DATAINIZIO, VRDOCID FROM [dbo].[TDPreventivi]  GROUP BY VRDOCID) MINDATA 
INNER JOIN
(SELECT MAX(VRDATA) AS DATAULTIMA, VRDOCID FROM [dbo].[TDPreventivi]  GROUP BY VRDOCID) MAXDATA  ON MINDATA.VRDOCID = MAXDATA.VRDOCID