Hello there,
I am currently trying to do the same : fine-tune Roberta on a very specific vocabulary of mine (let’s say : biology stuff).
About your first question, you should at least add some new  words, specific to your vocabulary, in  the Tokenizer vocabulary. See this discussion :  how can i finetune BertTokenizer? · Issue #2691 · huggingface/transformers · GitHub 2
Considering the MLM training, what class did you use exactly ? I am looking for more info online, found this (NLP-with-Deep-Learning/fine_tuning_bert_with_MLM.ipynb at master · yash-007/NLP-with-Deep-Learning · GitHub 6) but wonder how this would work for Roberta.
Thanks