The solution I have for the moment is, if I need, say Python 3.9, I do: pip3.9 install virtualenvwrapper and then create the env.

virtualenv accepts a --python argument. You should be able to add --python=/path/to/interpreter to your command line to mkvirtualenv and have it pass through.

To be honest, I never used virtualenv per se, always virtualenvwrapper. I tried to look for an option but when I do mkvirtualenv --help, I see mentions to what you say, but I frankly didn't try because, otherwise, I love virtualenvwrapper precisely because it's simple and straightforward to use.

I see that this is a very old discussion, but since it's still open and just in case someone else wonders if this is possible:

mkvirtualenv -a /path/to/your/project/folder -p python3.8 <new_env_name>
mkvirtualenv --python=/path/to/python3.8 <new_env_name>