Jean:
You can see the problems with relying on those Vendor statistics. The accuracy of the figures is entirely dependent on when you run the Year-End or Period-End process. If you are disciplined and run it exactly on time, the figures are accurate, but you still have the failing that you can't see past periods.
Depending on what you are trying to track, an alternative is to use the "Vendor Transactions" report which can be run with a Date Range. Remember to choose one of the "Current and hist." options for "Format". Then put check-marks on "Fully paid transactions" and, probably, "Summary Only" (your choice on the latter). These will be tax-in figures.