See https://chromium-review.googlesource.com/c/chromium/tools/build/+/991120 where we tried to run chromite's cros chrome-sdk tool in an environment that's purely vpython (ie: system python is ignored/not used). Doing so fails with the import error in:
https://logs.chromium.org/v/?s=chromium%2Fbuildbucket%2Fcr-buildbucket.appspot.com%2F8950327991836591184%2F%2B%2Fsteps%2Fgclient_runhooks__with_patch_%2F0%2Fstdout

It's failing despite oauth2client being checked in directly to chromite:
https://chromium.googlesource.com/chromiumos/chromite/+/master/third_party/oauth2client/

It seems vpython and chromite's virtual env/importer (maybe https://codesearch.chromium.org/chromium/src/third_party/chromite/scripts/wrapper.py?rcl=c23684514c93ed552d280b25b1280738eb42acfc&l=23 ?) aren't compatible. (work around is something like https://codesearch.chromium.org/chromium/build/scripts/slave/recipe_modules/chromite/api.py?rcl=4e63fe1d11d7df75f549da7d9cfef64cbdd05ff1&l=258)

Feel free to dupe this into another bug if one is already on file.