conditional vlookup

I am looking to have a conditional look up. Sample file is attached here.

I have three sheets in the file. Sheet 1 is where i want to capture last purchase date of an item code, which will be taken from sheet 2008 first, if not available for that item, then it will go to sheet 2007. If it is not available in both of the sheets, then I want the cell to be displayed with "-" & not "#N/A".
In the last column I want to calculate the number of days from last purchase date to 31-12-08.

thanks in advance