from typing import Sequence


def list_dict_values(dict_list: Sequence[dict], key: str) -> list:
    """
    Return a list of all unique values for a given key in a list of dictionaries.

    Example usage:
    ```
    mylist = [
        {
            'account_id': '1234567890',
            'vpc_id': 'vpc-123',
            'region': 'us-west-2'
        },
        {
            'account_id': '3456789012',
            'region': 'us-west-1'
        },
        {
            'account_id': '3456789012',
            'vpc_id': 'vpc-456',
            'region': 'us-west-1'
        }
    ]
    list_dict_values(mylist, 'vpc_id')
    > ['vpc-123', 'vpc-456']
    ```

    :param dict_list: A list of dictionaries.
    :type dict_list: list
    :returns: A list of unique values for a given key from a list of dictionaries.
    :rtype: list
    """
    return [x for x in list(set(y.get(key, None) for y in dict_list)) if x is not None]