Wait, you're selecting documents that are simultaneously stamped after September 26th and before August 27th? Only if they do that quantum time travel.
UPDATE: Sorry, I guess that was more of a snarky comment than solution. Here is code that should get you your results (swapped your operators):
select * from Purchase v
where cast( v.DocDate as datetime) <= cast('2014-09-26 12:00:00.000'as datetime )
and cast( v.DocDate as datetime) > cast('2014-08-10 12:00:00.000' as datetime)