I base my marriage license sources on the Civil registrations shown on page 90 in the Mills Book.

>> I have a photocopy of a Registration of Marriage record which I printed from microfilm at the Wisconsin Historical Society. The microfilm is for Vernon County marriages.<<

What exactly was microfilmed? Was it the actual marriage register from Vernon County or was a compilation done by another organization or individual?

If it was a film of the register, then you treat it as though you were working with the original ledger. The title would be whatever was on the cover and the author would be Vernon County Clerk. If it was actually a film of a compilation, the author should be whoever made the compilation with all the normal publication information.

In either case, the source record should indicate that the medium is microfilm located at the Wisconsin Historical Society.

Once you have a master source, you can create a citation from that source that includes the individual's name, book and page number, and/or the license number.

>>Her maiden name was Mary DeWitt. Where/how is the best place to note this--as part of the citation text in square brackets?<<

Square brackets are generally used to indicate infomation that is not included in the original so your idea would be appropriate. Be sure to note in the citation exactly what is written in the book though. In some cases, that might include the title "Mrs."