Managed to identify the relevant vocab.txt files for distilbert. Think having the vocab.txt related to a model, should also form part of the listed files.
In [4]: tokenizer.pretrained_vocab_files_map
Out[4]: 
{'vocab_file': {'distilbert-base-uncased': 'https://s3.amazonaws.com/models.huggingface.co/bert/bert-base-uncased-vocab.txt',
'distilbert-base-uncased-distilled-squad': 'https://s3.amazonaws.com/models.huggingface.co/bert/bert-large-uncased-vocab.txt',
'distilbert-base-cased': 'https://s3.amazonaws.com/models.huggingface.co/bert/bert-base-cased-vocab.txt',
'distilbert-base-cased-distilled-squad': 'https://s3.amazonaws.com/models.huggingface.co/bert/bert-large-cased-vocab.txt',
'distilbert-base-german-cased': 'https://s3.amazonaws.com/models.huggingface.co/bert/distilbert-base-german-cased-vocab.txt',
'distilbert-base-multilingual-cased': 'https://s3.amazonaws.com/models.huggingface.co/bert/bert-base-multilingual-cased-vocab.txt'}}