00-output.txt

vars_secret_funky_json: !vault |
  $ANSIBLE_VAULT;1.2;AES256;alan_host
  35356666616633303337313766346562613961313262333530663432393965303736653334306433
  6239666265343936343462653836386162343234353961330a306665396665353364613863316362
  66646663313737393763383565333237316663666339623063646666646261643338616261633330
  3634313634666264620a383632386661653330326435633861333031643334643237366430313733
  3733

{
    "vars_secret_funky_json": {
        "__ansible_vault": "$ANSIBLE_VAULT;1.2;AES256;alan_host\n35356666616633303337313766346562613961313262333530663432393965303736653334306433\n6239666265343936343462653836386162343234353961330a306665396665353364613863316362\n66646663313737393763383565333237316663666339623063646666646261643338616261633330\n3634313634666264620a383632386661653330326435633861333031643334643237366430313733\n3733\n"
    }
}

ansible_vault_yaml_json_conv.py

import json
import yaml


YAML_DATA = '''
vars_secret_funky_json: !vault |
    $ANSIBLE_VAULT;1.2;AES256;alan_host
    35356666616633303337313766346562613961313262333530663432393965303736653334306433
    6239666265343936343462653836386162343234353961330a306665396665353364613863316362
    66646663313737393763383565333237316663666339623063646666646261643338616261633330
    3634313634666264620a383632386661653330326435633861333031643334643237366430313733
    3733
'''

JSON_DATA = '''
{
    "vars_secret_funky_json": {
        "__ansible_vault": "$ANSIBLE_VAULT;1.2;AES256;alan_host\\n35356666616633303337313766346562613961313262333530663432393965303736653334306433\\n6239666265343936343462653836386162343234353961330a306665396665353364613863316362\\n66646663313737393763383565333237316663666339623063646666646261643338616261633330\\n3634313634666264620a383632386661653330326435633861333031643334643237366430313733\\n3733\\n"
    }
}
'''


class VaultData:
    def __init__(self, value):
        self.value = value


class VaultDecoder(json.JSONDecoder):
    def __init__(self, *args, **kwargs):
        kwargs['object_hook'] = self.object_hook
        super(VaultDecoder, self).__init__(*args, **kwargs)

    def object_hook(self, pairs):
        for key in pairs:
            value = pairs[key]
            if key == '__ansible_vault':
                return VaultData(value)

        return pairs


class VaultEncoder(json.JSONEncoder):
    '''
    Simple encoder class to deal with JSON encoding of Ansible internal types
    '''
    def default(self, o):
        if isinstance(o, VaultData):
            return {'__ansible_vault': o.value}
        return json.JSONEncoder.default(self, o)


def vault_representer(dumper, data):
    return dumper.represent_scalar(u'!vault', data.value, style='|')


yaml.add_representer(VaultData, vault_representer)


def vault_constructor(loader, node):
    value = loader.construct_scalar(node)
    return VaultData(value)


yaml.add_constructor(u'!vault', vault_constructor)


jdata = json.loads(JSON_DATA, cls=VaultDecoder)
print(yaml.dump(jdata, default_flow_style=False))

ydata = yaml.load(YAML_DATA)
print(json.dumps(ydata, cls=VaultEncoder, indent=4))

This is the best documentation of a working example I've been able to find. As such I've included it as a reference to ansible/ansible#44829.