I have upgraded from Visual Studio 2017 to 2019 (Community Edition). I also have installed the 'Python development' workload from the Visual Studio 2019 installer.
To add a virtual environment to a Python Project in Visual Studio, I right-click 'Python Environments' within the Python project (in the Solution Explorer View) and choose 'Add Environment...'. I opt for 'Virtual Environment', receiving the message:
"You will need to install a Python Interpreter before creating a virtual environment."
I have installed both the Anaconda 2019.03 and Python 3.7 interpreter, and I have access to both of them in Visual Studio.