When you make filter 01-08-12..31-12-12 (Net Change (LCY)), this represent "Net Change" in "Vendor - Trial Balance" report.
If you make filter ..31-12-12 (Net Change (LCY)), this will represent "Ending Balance" in the same report.
After that, you will be sure where is mistake and you need to compare this results with entries in "Detailed Vendor Ledg. Entry" table. You have to check and compare all the fields in the table.