The birth in VT at 1780 is early for by family. The name Isacc appears in this time period in my VT family. Ephraim Johnson, b. March 31, 1742 in Andover MA; m. Mary FARLEY of Reading,MA 1765. Andover is the family home for this Johnson family since 1646, and as the youngest (without an inheritance or land)moved iinto the unsettled American wilderness to Wilton NH. I know of only two of Andrew and Mary's children, Mary his dau that took care of them in their old age and my ancestor, Andrew who moved to Ludlow VT. Of Andrew's Children, all born in Ludlow VT, I have two males Isaac b. 1812 in Ludlow and David (my gggrandfather). My notes from the family genealogy show that Isaac's family(just children? parents too?) moved to Cinncinati OH. His children were: Charles, Lyman, Samuel and Emma. Davids' children were: Issace, ElizA, Hiram, Samuel, and Darius.
speculation: I think it is likely that Andrew Johnson had a brother Isaac to match the time period you are reseaching.