Hello Nick
Yes I can see the problem you have here, I mean she is almost certainly linked to this family because of the rarity of the name, but it is proving that link without a baptism isn't it.
I can see why you think she may be a daughter to David John Vernezobre and Eleanor/Nelly Sullivan.
But I see two problems against that. Firstly the Ellen Vernezobre on the 1851 Census (who I agree with her daughter Hannah Burke/Vernezobre being born in St George in East at same time your looking at, is most likely Madeliane Henriette mother too) would be too young to be the same person as the Nelly Sullivan who married David John. Also she is living on a pension from the East India Company, persumably from her widowed husbands service. David John Vernezobre does not have any record of service with them.
The only record with the East India Company is for a "John Daniel Abraham Vernezobre" who was in the companies service from 1775 - 1799 (So must have been born circa 1750-55). He would therefore seem the only possible candidate for Ellens deceased husband, and therefore the likely father of both Hannah and Madeliane Henrietta. It might be worth ordering his service record, as it may contain details of his family (never had one myself so can't say for certain what they do or don't contain).
Can't find a baptism for him either, but looking at Google Books I can see part of Daniel Vernezobres will of 1778 and he was definently a grandson to him:
"Vernezobre, Daniel of Great St Helens, merchant, my vault in parish of Christ Church Middlesex, dear wife.................(missing)........................Petersburgh,
160 pounds between my grandchildren John Daniel Abraham Vernezobre, David Vernezobre, Magdelen Henriette Vernezobre, and Henriette Frances Vernezobre; Executors: son John Daniel Vernezobre and friend Peter Guinard.
Dated 20 Nove 1776 (Mark of Daniel Vernezobre) Probate 20 Aug 1778.
Further administration 14 May 1791 to Henriette Vernezobre, spinster, Administratrix to John Daniel Vernezobre, deceased, Peter Guinard, the surviving executor also dying intestate. Daniel Vernezobre was formerly of Mile End Green, but late of Stratford Green."
I believe from the limited I can read of Bems article (again via Google books preview facility), that the middle son of Daniel, John Daniel never married and died single in 1780. So John Daniel Abraham Vernezobres father must surely have been the oldest son Charles Daniel (b.1726) or the youngest David Jean (b.1737). Puzzling why there is no baptisms to be found for any of them. Perhaps they were using a very small independant chapel, the records of which are not publicly held or available? I have heard there are some registers of very small London huguenot chapels held in private hands.