Jacob Deckard Jr wife's names is confusing in the records I have as well. One reads Mary Vance and another reads Mary Elizabeth Vance.
She (Mary Vance) belonged to the Vance family of Washington County (at the time of her marrage, it was Botetcourt County), including Samuel Vance, colonel in the Revolutionary Army. Jacob was on active duty as private in American Revolutionary Army on shipping by the following.
War Department at Washington, D.C. states under the date of May 23, 1921 that Jacob Deckard, rank not stated, served as a soldier of Virginia. This record also has a paragraph which reads:
"Recorder of Deeds Office of Wythe County, in Book VII, page 2221, shows Jacob Deckard gave a deed on February 13, 1821; page 709 shows Jacob Deckard & wife Elizabeth (possibly Mary Elizabeth), of Wythe County, Virginia, together with their son, Jenrhy Deckard (see 4009 & his wife Elizabeth, of Wythe County, sold a tract of 85 acres of land in that county on May 31, 1828"
"Also Book IX, page 633, shows Jacob Deckard gave a deed to his son Henry on August 28, 1824. Also Book XII, page 147, shows Jacob & Wife Elizabeth gave deed dated September 30, 1831.

I know this is rather lengthy, but it shows the Jacob and wife Elizabeth and the possible Mary Elizabeth as uncertain.