The vendor bill amount is always the full amount of the bill (for example, $100). The bill balance is $0 once the payable portion has been fully addressed, as the withheld tax is treated as a liability rather than an outstanding payable.
Reports alway display the full vendor bill amount. The withheld tax is not deducted from the bill value shown on the standard reports.
Note: A dedicated Tax Withholding report summarizing tax withheld per vendor and net paid might be considered in a future release, but is not currently available.