For your unable to find the TargetArguments path issue, the parameter value for TargetArguments is not correct
Change it to: /TargetArguments="$(Build.SourcesDirectory)\xxxxxx.AD.UnitTest\bin\Release\VonExpy.AD.UnitTest.dll"
Now for the xUnit test, according to this thread: Error encountered when running dotcover console runner using xunit as the target executable, you hsould use xunit.console.exe in xunit.runner.console package.