Creating python virtual environment in Visual Studio fails: returned non-zero exit status 1
Turns out this was a known issue with previous versions of Anaconda.  The Python 3.6 64-bit interpreter that was listed as giving an error was actually an Anaconda version of Python as well.  
Simply using the package manager to upgrade to the most recent versions of Anaconda fixed the issues.
In Visual Studio 2015 and 2017, I always get an error when trying to create a virtual environment with certain base interpreters.   
Base interpreters that work:

Python 3.6 32-bit
Python 2.7, 64-bit
Anaconda 5.0.1 (2.7, 64-bit)

Base interpreters that give error:

Anaconda 5.0.1 (3.6, 64-bit)
Python 3.6 64-bit

The error:

Error: Command '['F:\OneDrive\Visual Studio 2017 Projects\Web Test\DjangoWebProject1\DjangoWebProject1\env4\Scripts\python.exe', '-Im', 'ensurepip', '--upgrade', '--default-pip']' returned non-zero exit status 1.
  Virtual environment was not created at 'F:\OneDrive\Visual Studio 2017 Projects\Web Test\DjangoWebProject1\DjangoWebProject1\env4'. Exit code: 1
  Virtual environment was not created at 'F:\OneDrive\Visual Studio 2017 Projects\Web Test\DjangoWebProject1\DjangoWebProject1\env4'