Adding the vault charm to the bug:
2019-08-12 10:42:44 INFO juju-log Invoking reactive handler: reactive/vault_handlers.py:128:snap_install
2019-08-12 10:42:45 INFO juju-log Installing core from store
2019-08-12 10:44:57 ERROR juju-log Hook error:
Traceback (most recent call last):
  File "/var/lib/juju/agents/unit-vault-0/.venv/lib/python3.6/site-packages/charms/reactive/__init__.py", line 73, in main
    bus.dispatch(restricted=restricted_mode)
  File "/var/lib/juju/agents/unit-vault-0/.venv/lib/python3.6/site-packages/charms/reactive/bus.py", line 390, in dispatch
    _invoke(other_handlers)
  File "/var/lib/juju/agents/unit-vault-0/.venv/lib/python3.6/site-packages/charms/reactive/bus.py", line 359, in _invoke
    handler.invoke()
  File "/var/lib/juju/agents/unit-vault-0/.venv/lib/python3.6/site-packages/charms/reactive/bus.py", line 181, in invoke
    self._action(*args)
  File "/var/lib/juju/agents/unit-vault-0/charm/reactive/vault_handlers.py", line 133, in snap_install
    snap.install('core')
  File "lib/charms/layer/snap.py", line 53, in install
    _install_store(snapname, **kw)
  File "lib/charms/layer/snap.py", line 303, in _install_store
    out = subprocess.check_output(cmd, stderr=subprocess.STDOUT)
  File "/usr/lib/python3.6/subprocess.py", line 356, in check_output
    **kwargs).stdout
  File "/usr/lib/python3.6/subprocess.py", line 438, in run
    output=stdout, stderr=stderr)
subprocess.CalledProcessError: Command '['snap', 'install', '--channel=stable', 'core']' returned non-zero exit status 1.

2019-08-12 10:44:57 DEBUG install Traceback (most recent call last):
2019-08-12 10:44:57 DEBUG install   File "/var/lib/juju/agents/unit-vault-0/charm/hooks/install", line 22, in <module>
2019-08-12 10:44:57 DEBUG install     main()
2019-08-12 10:44:57 DEBUG install   File "/var/lib/juju/agents/unit-vault-0/.venv/lib/python3.6/site-packages/charms/reactive/__init__.py", line 73, in main
2019-08-12 10:44:57 DEBUG install     bus.dispatch(restricted=restricted_mode)
2019-08-12 10:44:57 DEBUG install   File "/var/lib/juju/agents/unit-vault-0/.venv/lib/python3.6/site-packages/charms/reactive/bus.py", line 390, in dispatch
2019-08-12 10:44:57 DEBUG install     _invoke(other_handlers)
2019-08-12 10:44:57 DEBUG install   File "/var/lib/juju/agents/unit-vault-0/.venv/lib/python3.6/site-packages/charms/reactive/bus.py", line 359, in _invoke
2019-08-12 10:44:57 DEBUG install     handler.invoke()
2019-08-12 10:44:57 DEBUG install   File "/var/lib/juju/agents/unit-vault-0/.venv/lib/python3.6/site-packages/charms/reactive/bus.py", line 181, in invoke
2019-08-12 10:44:57 DEBUG install     self._action(*args)
2019-08-12 10:44:57 DEBUG install   File "/var/lib/juju/agents/unit-vault-0/charm/reactive/vault_handlers.py", line 133, in snap_install
2019-08-12 10:44:57 DEBUG install     snap.install('core')
2019-08-12 10:44:57 DEBUG install   File "lib/charms/layer/snap.py", line 53, in install
2019-08-12 10:44:57 DEBUG install     _install_store(snapname, **kw)
2019-08-12 10:44:57 DEBUG install   File "lib/charms/layer/snap.py", line 303, in _install_store
2019-08-12 10:44:57 DEBUG install     out = subprocess.check_output(cmd, stderr=subprocess.STDOUT)
2019-08-12 10:44:57 DEBUG install   File "/usr/lib/python3.6/subprocess.py", line 356, in check_output
2019-08-12 10:44:57 DEBUG install     **kwargs).stdout
2019-08-12 10:44:57 DEBUG install   File "/usr/lib/python3.6/subprocess.py", line 438, in run
2019-08-12 10:44:57 DEBUG install     output=stdout, stderr=stderr)
2019-08-12 10:44:57 DEBUG install subprocess.CalledProcessError: Command '['snap', 'install', '--channel=stable', 'core']' returned non-zero exit status 1.
2019-08-12 10:44:57 ERROR juju.worker.uniter.operation runhook.go:132 hook "install" failed: exit status 1