I'm not sure. Currently the vendor folder is not set in stone (unlike .git and node_modules) so someone could use a different folder name for the vendor folder.

FWIW, the current way to ignore it would be to do the following in a deno.json (a little verbose... it might be nice if we had a more global way to ignore folders or files):

{
  "lint": {
    "files": {
      "exclude": ["vendor/"]
    }
  },
  "fmt": {
    "files": {
      "exclude": ["vendor/"]
    }
  },
  "test": {
    "files": {
      "exclude": ["vendor/"]
    }
  },
  "bench": {
    "files": {
      "exclude": ["vendor/"]
    }
  }
}

@dsherret see #15300