LINT_PATHS in `noxfile.py` doesn't include `vertexai`

It looks like the relatively new vertexai source folder is missing from `LINT_PATHS`. As a consequence it's not linted properly automatically and the codebase has now some lint errors. Suggestion is to update the `LINT_PATHS` variable and fix the formatting for `vertexai/language_models/_distillation.py` and `vertexai/vision_models/_vision_models.py`. This should be a trivial fix.