You have 2 options. 1, you can use a dreamcast sd card adapter and the program "DreamShell" to transfer the vmu saves that you have to a sd card which you can then transfer to a pc.
2, You can use a dreamcast program called "Dream Explorer 0.8.5" to copy vmu game saves directly to a pc. With option 2 though it has pretty much every dreamcast game save ever made so once you see that you might not care about backing up vmu saves anymore.