More information from Exeter baptismal records strongly suggests that Mary Jane Vial's grandfather and Mary Tremlett's father was probably Samuel Tremlett, Junior, born before 1720 and most probably the eldest son of Samuel Tremlett (1690 - 1765) and his wife Elizabeth (d. 1773). The baptism register of the Exeter Non-Conformist congregation of Rev. Jos. Hallett Jnr records the baptism, on 13th March 1740, of Mary, daughter of Samuel Tremlett Jnr and his wife Mary. The register was commenced in 1720, and the first recorded baptism of a child of Samuel and Elizabeth was of a daughter in 1722, so if Samuel Jnr is indeed their eldest son, his birth would have been before 1720.
The collection Wills and Administrations Proved in the the Bishop of Exeter, 1559-1799 contains reference to the will of a Samuel Tremlett who died in 1755. This is consistent with the absence of references of Mary's father in the wills of Samuel Snr and other family members.
This new information has been included in the page about Mary's family: her family tree has also been updated.