There are a number of great similarities that you have already identified in the question above. It is also important, however, to consider the differences between Renfield and Nancy. What is clear about Nancy is that she is a character who Dickens meant the reader to pity. She at various points in the novel rails against the life that she has been forced to lead, and is allowed, in her own words, to speak of the situation she finds herself in, concerning her relationship with Bill Sikes:
When such as me, who have no certain roof but the coffin-lid, and no friend in sickness or death but the hospital nurse, set our rotten hearts on any man, and let him fill the place that parents, home, and friends filled once, or that has been a blank through all our wretched lives, who can hope to cure us?
Nancy, in some ways, is one of the most complicated characters in the entire novel. She is used to present a very grim and rather unyielding view of grace and redemption in the novel: once you are too far down the road of crime, apparently it is impossible to turn back, and her death seems to reinforce this. However, in spite of this, she is still able to show genuine goodness through her actions on behalf of Oliver.
Nancy is allowed to speak out on her own behalf, and the reader has real empathy for her situation as a result. The difference between Renfield and Nancy is that the reader only ever finds out about Renfield through the notes of Dr. Seward in his journal. There is nothing about him as a character that is not mediated. In addition, whereas Nancy is described as good and wanting to try and help Oliver, Renfield's actions and description creates something of a mystery. He is characterised as a "lunatic" and a "madman" by the other characters, and his actions are always suspect. Even when he tries to prevent Mina from returning to Dracula, it is unclear whether it is out of goodness, or whether he selfishly wishes for immortality himself.