The decoded data ("decoded" from the perspective of the program using the data) is arbitrary-depth arbitrary-length lists-of-lists-of-...-binaries (lists and binaries are allowed to be empty). The encoded data is of course a single flat binary string.

It has been tested against Ethereum's official Python implementation.

You can see the methodology in <tests/vrlp_testgen.erl>:

10,000 decoded data structures are generated at random
the test generator calls vrlp:encode/1 to generate an encode/decode pair
assert that encode -> decode round trips properly in the vrlp module
print out an encode-decode pair as a python term to stdout
python program tests the encode-decode pairs against Ethereum's library

The heart of the methodology is here:

% input: decoded_data
% format a case as
% {'decoded_data': <python term for rlist>,
%  'encoded_bytes': bytes([B1, B2, B3, ...])}
format_case_py(DecodedData_rlist) ->
    % EncodedData_bytes = rlp:encode(
    % DD_js = format_data_js(DecodedData_rlist),
    EncodedData_bytes          = vrlp:encode(DecodedData_rlist),
    % assert encode -> decode round trips
    {DecodedData_rlist, <<>>}  = vrlp:decode(EncodedData_bytes),
    EncodedBytes_py   = format_bytes_py(EncodedData_bytes),
    DecodedData_py    = format_data_py(DecodedData_rlist),
    ["    {'decoded': ", DecodedData_py, ",\n",
     "     'encoded': ", EncodedBytes_py, "}"].

The Python side:

def main():
    all_encode_work = True
    all_decode_work = True
    for case in cases:
        decoded_data       = case['decoded']
        encoded_bytes      = case['encoded']
        real_encoded_bytes = rlp.encode(decoded_data)
        real_decoded_data  = rlp.decode(encoded_bytes)
        # check if encode/decode works
        encode_works = (real_encoded_bytes == encoded_bytes)
        decode_works = (real_decoded_data == decoded_data)
        print('encode works: %s' % (encode_works))
        print('decode works: %s' % (decode_works))
        # update globals
        all_encode_work = all_encode_work and encode_works
        all_decode_work = all_decode_work and decode_works
    # print globals
    print('all encode work: %s' % (all_encode_work))
    print('all decode work: %s' % (all_decode_work))