It seems vpython will attempt to create 2 dirs at run-time, .vpython-root and .vpython_cipd_cache.

The location of .vpython-root can be configured via a cmd-line arg or env var:
https://codesearch.chromium.org/chromium/infra/go/src/go.chromium.org/luci/vpython/application/application.go?rcl=3c7067507989e558bc88a523ff2fff6f6c4266bb&l=46

However, .vpython_cipd_cache seems to be hard coded under the home dir and can't be told to go elsewhere. This is problematic because for some platforms (eg cros), the home dir isn't necessarily writable. For instance, I'm seeing this failure:

localhost /usr/local/tmp/bpastene # VPYTHON_VIRTUALENV_ROOT=/tmp ./vpython
[E2018-05-15T12:23:32.498188-07:00 7129 0 client.go:1431] Failed to install infra/python/virtualenv:d7fe4ca0377e894777e4f0a4d10defc600baf537 - mkdir /root/.vpython_cipd_cache: read-only file system
[E2018-05-15T12:23:32.498726-07:00 7129 0 annotate.go:242] original error: failed to update packages, see the log
[E2018-05-15T12:23:32.498771-07:00 7129 0 annotate.go:242] 
[E2018-05-15T12:23:32.498796-07:00 7129 0 annotate.go:242] goroutine 1:
[E2018-05-15T12:23:32.498816-07:00 7129 0 annotate.go:242] #0 go.chromium.org/luci/cipd/client/cipd/client.go:122 - cipd.init()
[E2018-05-15T12:23:32.498849-07:00 7129 0 annotate.go:242] #1 go.chromium.org/luci/vpython/./<autogenerated>:1 - cipd.init()
[E2018-05-15T12:23:32.498865-07:00 7129 0 annotate.go:242] #2 ./<autogenerated>:1 - main.init()
[E2018-05-15T12:23:32.498884-07:00 7129 0 annotate.go:242] #3 runtime/proc.go:186 - runtime.main()
[E2018-05-15T12:23:32.498899-07:00 7129 0 annotate.go:242]   annotation #0:
[E2018-05-15T12:23:32.498918-07:00 7129 0 annotate.go:242]     reason: failed to create new VirtualEnv
[E2018-05-15T12:23:32.498932-07:00 7129 0 annotate.go:242]   annotation #1:
[E2018-05-15T12:23:32.498946-07:00 7129 0 annotate.go:242]     reason: failed to download packages
[E2018-05-15T12:23:32.498960-07:00 7129 0 annotate.go:242]   annotation #2:
[E2018-05-15T12:23:32.498978-07:00 7129 0 annotate.go:242]     reason: failed to download packages
[E2018-05-15T12:23:32.498994-07:00 7129 0 annotate.go:242]   annotation #3:
[E2018-05-15T12:23:32.499017-07:00 7129 0 annotate.go:242]     reason: failed to install CIPD packages
[E2018-05-15T12:23:32.499046-07:00 7129 0 annotate.go:242] 
[E2018-05-15T12:23:32.499066-07:00 7129 0 annotate.go:242] #4 runtime/asm_amd64.s:2361 - runtime.goexit()