The original VBA emulator is ancient and has ceased development in 2004. VBA-M is a continuation of VBA, as it is regularly updated.
VBA-Next is an optimized version of VBA with speedhacks and is available as a libretro core for RetroArch.
For the Wii, it's better to use the VBA-Next core of RetroArch than to use VBA-GX. However, VBA-GX supports using the Wii Remote as a gyrometer (required for a few GBA games), whereas VBA-Next does not.