I got this to work by following the guide here:
And also example code from vvvsample here:
There is only one caveat: I have to copy the COM dll to the same directory as WinUnit.exe.
I am currently passing the full path of the external manifest and the full path of the test directory to CActCtxHandle.Create() (ex. "C:\MyCppTests\Release\MyCppTests.dll.manifest" and "C:\MyCppTests\Release" respectively).
The COM dll also resides in "C:\MyCppTests\Release" but WinUnit.exe resides in a different directory (ex. "C:\WinUnit").