While it is true that the elder of the three sons of William Abernathy of Mecklenburg County Virginia (David) does, apparently, come remarkably close to matching the deduced age of the David Wilson Abernathy found in Jefferson County Alabama in 1830 and 1840, there is nothing of which I currently know to link the two. Nor does it explain where these siblings might have been between 1800 and 1825, when Samuel Vance Abernathy is reputed to have married his presumed cousin, Eleanor Abernathy, in Alabama.
Also, there are no Vance families represented on the 1830 and 1840 census enumerations of Jefferson County, AL. Rocketts, yes. Abernathys, yes. But no Vance Families. Unless, of course, they're mis-labeled as "Vines" families.
Rather than make wild speculations, a better approach at this point in time might be to utilize Y-DNA testing to establish if a relationship exists between one or more male descendants of David Wilson Abernathy of Jefferson County AL and the various other Abernethy/Abernathy lines for which Y-DNA data is now beginning to accumulate.