Try...
=IF(ISERROR(VLOOKUP(A1,'[June Inv File.xls]Sheet1'!$A:$C,3,0)),0,VLOOKUP(A1,'[June Inv File.xls]Sheet1'!$A:$C,3,0))
Replace the 0 with whatever you want to be shown if A1 cannot be found in the other sheet.
Drag the formula down for each A you want to search for.
If you are in excel 2007 or later there is the ISERROR function that negates the need for the IF function.
Regards
Roger