I'm just speculating since I don't own a Wii and can check this out. But from what I gather, there should be a folder like /vbagx/save on the SD card. If this file is exactly 128 kiB big (131072 bytes) it's likely a sav file with a different file extension. If so, just copy it to the computer and rename the extension to sav, so you might have a pair of files like lsdj.gb and lsdj.sav.
If the file is not 131072 bytes big, it might be a save state file. This might be what the autosave file you're talking about is. Ie, a snapshot of the running Gameboy so you end up right where you left off last time. Does that sound familiar with how VBA GX works? If this is the case, you may want to load the atosave file as described in the previous post, in VBA on the PC, as a save state, or SGM file.
If all that fails, give me whatever save file you have and I'll see if I can extract your data.