Tom...
I saw several Alabama buttons, including the "MVCs" displayed on the web site at: www.relicman.com/buttonCSA.htm. They looked great- interesting that they were apparently made in Connecticut (Scoville Mfg.Co.) and New York (Horstmann & Allien).
As far as I know there was never a unit at Gaine's Mill or in the Army of Northern Virginia called the "Mobile Volunteer Corps." I am just guessing...but the buttons probably belong to one of the early Alabama Regiments that went to Virginia and which had companies from Mobile. I would suggest the Wilcox Brigade (8th-9th-10th-11th-14th Alabama) or one of the other 4 or five regiments with companies from Mobile (12th, 7th, 5th AL Battallion, etc). I think the "Volunteer" refers to the status of the volunteer troops and not the unit itself.
I am sure one of the other contributors to this board probably has the specific answer about your buttons.
John