I was watching a video, and it used this command to invoke Vim:

$ vim -u NONE

The vim(1) says:

-u {vimrc}
Use the commands in the file {vimrc} for initializations. All the other initializations are skipped. Use this to edit a special kind of files. It can also be used to skip all initializations by giving the name NONE. See :help initialization within vim for more details.
-U {gvimrc}
Use the commands in the file {gvimrc} for GUI initializations. All the other GUI initializations are skipped. It can also be used to skip all GUI initializations by giving the name NONE. See :help gui-init within vim for more details.

And in :help initialization:

If Vim was started with -u filename, the file filename is used. All following initializations until 4. are skipped. vim -u NORC can be used to skip these initializations without reading a file. vim -u NONE also skips loading plugins.

For having a clean Vim for testing or secondary ~/.vimrc, you only need to use -u option. I seem to remember that I had done renaming vimrc just to make sure I don’t have user configuration to interfere, wish I had known or at least tried to find out about this option at the time.

There are 12 types of initializations listed, but this -u only covers the 3. and 4 if it’s NONE. But I think it’s more than enough to claim that the environment is clean or vanilla.