Maybe she was just some floozie Elros picked-up in some bar on one of his journeys and she had no pedigree, but he fell head over heels in love with her.
Or maybe her father turned out to be a serial killer or child molester or maybe after the birth of Vardamir she ran off with a drummer from Dubuque, so for whatever reason, her name and pedigree were expunged from the records, because, as you say, they were very big on the genealogical records of their Kings and it should have been listed there.
Probably Tolkien just felt her name and background were insignificant, when compared that of Elros, that he never got around to working it out and putting it down on paper leaving us with this quandary.