Take it from me as someone who has experienced the J/C product on both airlines multiple times this year - switching MAN-ATL from DL to VS is a backward step.
I would also be concerned about arriving on VS and transferring at ATL (as most do) as it becomes in effect inter-lining.
I have interlined between a DL national flight and a VS international flight as recently as two months ago and it was an irritating experience as the VS gate crew couldn't get my DL issued boarding pass to work on the scanner.
Priority boarding had gone right out of the window by the time they had sorted it.
I'll be avoiding the risks - like I say, I'll be looking for alternative routings after the switch.