"Vengeance is Mine" is a novel - one I have not read so I can only speak in general terms. The great thing about novels is that you can make stuff up.I know of no evidence that Jane Rochford had a child by Thomas Cromwell. It seems quite unlikely to me that these two would have an intimate relationship.
As far as I've read, Jane Parker (Rochford) never had any children, with Cromwell or otherwise. The idea for that storyline may stem from the questions many have had as to how, exactly, Jane Rochford not only escaped the scandal that brought down her husband and sister-in-law, but also managed to maintain a place in royal service. While Phillippa Gregory's The Boleyn Inheritance uses Rochford's connection to Norfolk, this novel may be using the letter found in Cromwell's papers that Lady Rochford wrote asking for his assistance in maintaining or acquiring a pension of some kind. When her husband was executed, all his goods and lands reverted to the crown, leaving her destitute in comparison to the wealth she had before his fall.
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