That's never really seemed plausible to me. I mean if you think about it, that leaves only two possibilities:
1: Vo had a family before he returned and this family is the line that the Royals come from. The one problem with this is then you have to explain why the royal line has qualities in common with the Returned. How would that work if Vo had his kid(s) before he himself returned?
2. Vo conceived a child or children during the week he was alive after he returned. Possible, yes. Likely? Debatable.
Something about the story told to us so far doesn't add up...