Since some years have passed since your post, you may already have this data, but I'll give you what I have about Madison Boyd Donegan (Dunigan/Dunnegan/etc.)
He appears to be the son of William S. "Donegan". In the 1850 census of Dickson Co. TN, middle dist., no. 939: Wm. S. Dunigan 45 NC, Elisabeth 45, Charity 25, Margarett 23, James 21 schoolteacher, Mary 20, Standford 18, Martha 15, Wm. 11, Madison B. 9 m, Amanda J. 7, Louisa 3, all b. TN.This Madison B. born ca. 1841 is probably your Madison Boyd. If so, his parents seem to be Wm. S. Dunigan and Elizabeth _?_, possibly Elizabeth McCasland, who may have been married ca. 1825 in Maury Co. TN.She may be the dau. of Andrew McCasland, son of John McCasland Sr. (see Genealogy of McCasland by Thesta Kennedy Scogland, p. 23, which lists Elizabeth as dau. of Andrew; Scogland notes the above 1850 census record, but gives no data on Wm. S. Dunigan, and in fact seems to be guessing that his wife is Elizabeth McCasland).The surname could also be spelled McCaslin, and in fact that is the spelling given for Andrew McCasland in the 1850 census of Hardin Co. TN; the surname is spelled a number of other ways as well.I am being very tentative about the names and relationships here, since I personally have no specific proof of them.
I believe it was Madison Boyd Dunigan who married "Vina" Tidwell, as you stated. She was a daughter of Moses Tidwell and Nancy McCaslin (not McCasland - Nancy is a daughter of my ancestor John McCaslin by his 1st wife Ruth Andrews; if related to Andrew McCasland mentioned above, the relationship is not a close one).Tidwell sources list her as Vincy Tidwell b. 2 Mar 1853 Dickson Co. TN, d. 9 Sep 1876 Dickson Co. TN, mar. unk. Donegan (who I believe is your Madison Boyd Donegan).I have not verified these dates; they may be from a tombstone (or a Bible; some Tidwell Bibles have survived).If she was called Vina as you mentioned, my guess is that it was a nickname, and so perhaps her real name was Elvina or Lavina, both of which are found in related families.However, since Tidwell sources call her "Vincy", I think that an additional possibility for her name is Vin Cynthia.My 2g-gmother was Vin Cynthia Jane (Casey) Butler (b. 1847), whose maternal grandparents were the above John McCaslin and Ruth Andrews - so my 2g-gmother was a cousin of your Vina (Tidwell) Donegan.Letters proove that their families were in regular contact with one another, so it would not be surprising to find that the name Vin Cynthia was also used by your Tidwells.My 2g-gmother's paternal grandparents were Abner Casey and Vin Cynthia Jane Reavis, so you see that the name Vin Cynthia occurs twice in my family.It was sometimes spelled Vincynthia, but when my 2g-gmother signed her name, Vin and Cynthia were two separate words.I have seen this unusual name only in my family.You might look for your Vina Tidwell's signature on documents such as deeds, receipts in estate settlement records, etc.; when signing such records, she might have used her full name, or at least her initials and last name (look for the initials VC or VCJ).My 2g-gmother generally went by the name Jane, so if you find instances of your Vina (Tidwell) Donegan being called Jane, that might be another clue as to her real name.Alternatively, someone may have seen her name spelled "Viney" and misread it as "Vincy", in which case you can probably ignore all my discussion about the name Vin Cynthia.