The only img to vdi conversion know about, would be export of very old disk image formats from CloneCD and Virtuo CD Manager respectively. Tools like ISOBuster is probably your best choice for this matter as it supports numerous formats and most likely can be used for this as well.
If not, the only viable alternative would be to obtain the original software, in this case CloneCD and export the image to standard ISO, which you can then convert to any other disk image format.
Updated: September 15, 2023