Welcome to Rootschat Peter.
FreeREG has the 1789 Marriage and John VICARY is described as a Sojourner (temporary resident) on it.
Now I did notice a THOMAS/BETTY Vic* Baptising Children in Milverton at the same time as JOHN/JANE with the same names;'
EG;
JAMES, 25 April 1790 to John/Jane VICKRY
JAMES, 17 Apr 1791 to Thomas/Betty VICKRY
SARAH, 29 Oct 1797 to Thomas.Betty VICKRY
SARAH, 2 Dec 1798 to John/Jane VICARY
JOHN/JANE had James 1790/John 1794/Isaac 1796/Sarah 1798/George 1803/ Mary 1804/Ann 1806/Harriet 1807/Robert 1810
THOMAS/BETTY had Mary 1784/Betty 1786/James 1791/James 1792/Sarah 1797/ John 1801 and a Hannah 1788, Baptised 1807
On Marys Baptism their abode was Wiveliscombe*
As you say John was buried 1844 age 82 (birth c 1762)
Thomas 1802 age 40 (Birth c 1762)
Possible Thomas Baptised Milverton 1760 to JOHN/MARY
Another Child of theirs was James 1756
Although I think it is possible John/Thomas were Siblings I cant prove it as such.